Ich (white spot disease, caused by the parasite Ichthyophthirius multifiliis) usually shows as small, bright white, evenly scattered spots about the size of a grain of salt on the body and fins. Only a microscope confirms it, several other conditions look similar, and the white spot you can see is the one stage no treatment reaches. So the order is: confirm, then treat exactly as the product label or an aquatic veterinarian directs.

This page covers recognition, the look-alikes, why the treatment advice online disagrees with itself, and what you can safely do in the first hour. It does not give a dose, a temperature or a schedule, and that is deliberate.

Is It Ich? What White Spot on Fish Actually Looks Like

The aquariumscience.org ich page (read September 11, 2026) describes it precisely: "very well defined, very white spots only the size of a grain of salt, no larger," evenly spread over the fins and body, embedded in the skin so they "only slightly protrude," and they "almost never occur on the eyes."

The spots are not always the first sign. The University of Florida IFAS Extension fact sheet on white spot infections (FA006, by Ruth Francis-Floyd, Roy Yanong and Deborah Pouder, read September 11, 2026) lists signs that can come before the spots: irritation, flashing (rubbing against objects), increased mucus, weakness, loss of appetite and decreased activity. The Merck Veterinary Manual's chapter on parasitic diseases of fish (reviewed May 2022, updated June 2026, read September 11, 2026) adds lethargy, clamped fins and dark coloration, with white dots "often visible on the skin."

One version shows no spots at all. UF/IFAS warns that when the parasite infects the gills, "white spots may not be seen at all, but fish will die in large numbers," and the gills are often pale and swollen. Fish breathing hard or dying without visible spots is an emergency, not a wait-and-see situation. Our aquarium emergency guide gives the order of checks for exactly that moment.

Things That Look Like Ich

The same aquariumscience.org page names the common imitators, and they matter because the wrong identification leads to the wrong bottle.

aquariumscience.org also notes that Tetrahymena, seen mainly in livebearers, "can look exactly like ich, needing a microscope to differentiate it." That is the practical point of the table. It helps you describe what you see accurately. It cannot tell you what it is.

Frayed or disappearing fin edges are a different question altogether, and fin rot vs fin nipping covers how to tell damage from disease.

How it is actually confirmed

UF/IFAS states that "a diagnosis requires confirmation by identification of the parasite in infected tissue using a compound microscope," using skin scrapes and gill or fin clips. The Merck manual describes the parasite under 40x and 100x magnification as large, round, covered with cilia, with a "characteristic horseshoe-shaped macronucleus." That is a veterinarian's or a trained fish-health professional's job, and it is the step most home advice skips.

Why the Spot You Can See Is the Stage Nothing Reaches

This is the single most useful fact about ich, and it explains most failed treatments.

UF/IFAS describes three stages:

1. Trophont, the feeding stage. It burrows between the thin outer layers of the fish's skin. This is the white spot. UF/IFAS says it is "protected from chemical treatment" because it is covered by the fish's tissue and mucus.
2. Tomont, the reproducing stage. The mature parasite leaves the fish, settles on a surface and divides inside a cyst. Merck notes that "while encysted, parasitic life stages are refractory to chemical treatment, but cysts can be removed by thorough cleaning."
3. Theront, the free-swimming stage that hatches out and looks for a new host. UF/IFAS calls this phase "unprotected and, therefore, highly susceptible to chemicals."

So a treatment only works on the parasites that are swimming at the time it is in the water. The spots on your fish were not killed by it and could not have been. That is why medication labels run over a period of time rather than once, and why stopping early because the spots faded is the classic way an outbreak returns.

Temperature changes the speed. UF/IFAS gives the life cycle as roughly 3 to 6 days at 75 to 79 °F and much longer in cooler water. That figure describes the parasite's biology. It is not a treatment instruction, and nothing on this page tells you to change your tank's temperature.

The Honest State of the Treatment Advice

Search this topic and you will find confident, specific instructions that contradict each other. That is not because some writers are careless. The evidence really is uneven, and it is worth knowing where the disagreements sit. Status as of September 2026:

Heat. Much of the popular advice says to raise the temperature. aquariumscience.org's ich and heat page (read September 11, 2026) states that "there are several side by side scientific studies with controls which show heat is NOT effective," and argues that apparent success comes from other factors. We are not settling that argument, and we will not publish a heat protocol into it. Raising temperature also lowers the oxygen water can hold, which matters for fish whose gills are already under attack.

Salt. Widely recommended, and just as widely unsuitable for some animals. Several fish, and the shrimp and snails many people keep alongside them, react to salt concentrations other fish tolerate. This site publishes no salt amount for any purpose.

Medications. Commercial ich treatments exist and work on the free-swimming stage when used as labeled. The label is the authority: the amount, how long, what to do with chemical filter media, and above all which animals it is not safe for. Many ich products are not safe for invertebrates or for some sensitive fish. If the label does not address the animals in your tank, treat that silence as a no and ask before using it. Never combine products the label does not say can be combined.

What is not in dispute across every source read for this page: identification should be confirmed, the visible spot is protected, treatment has to cover the free-swimming stage, and prevention through quarantine beats all of it.

What You Can Safely Do in the First Hour

None of these steps is a treatment, and all of them help whoever treats.

• Look at every fish, not just the spotted one. Note which fish, where the spots are, and whether any fish is breathing hard, hanging at the surface or hiding.
• Test the water and write the numbers down. Ammonia, nitrite, nitrate, pH and temperature. Stress from poor water makes every parasite problem worse, and a vet will ask. Aquarium water parameters explains what each reading tells you.
• Take clear photos and a short video in good light, close to the glass. Spots are easier to judge on a screen than through a moving tank.
• Stop adding anything. No new fish, plants or decor, and do not move nets, siphons or buckets between tanks.
• Write down the timeline. When new fish or plants arrived, when the spots appeared, any recent changes. UF/IFAS notes that active disease often shows up one to three weeks after fish are shipped.
• Contact an aquatic veterinarian. Our guide to finding an aquatic veterinarian for fish covers what they do and how the visit works. The World Aquatic Veterinary Medical Association runs a Find a Fish Vet directory searchable by town and country (read September 11, 2026).

If a fish dies, remove it promptly and keep notes. It is useful evidence, not only a loss.

Preventing the Next Outbreak

Ich almost always arrives with something: a new fish, or water and plants carried over from another tank. The most effective control is keeping it out.

UF/IFAS recommends a minimum 30-day quarantine for new fish, and the Merck Veterinary Manual's management chapter calls 30 days "the minimum time for quarantine, but longer periods may be necessary" (both read September 11, 2026). That is a separate tank, separate equipment and patience. Our page on setting up a quarantine tank covers what that involves in practice.

Two rules that apply whatever you decide:

• Never release fish, plants or tank water into a pond, stream, lake or drain. Sick or unwanted aquarium animals and plants can carry disease to wild fish and can become invasive. Rehoming goes through a store, a club or a vet, never the outdoors.
• Keep equipment separate between a sick tank and a healthy one. Wet nets carry the free-swimming stage.

FAQ

Can I tell ich apart from other white spots just by looking? Only partly. Ich is typically bright white, salt-grain sized, round, evenly spread and almost never on the eyes, according to aquariumscience.org. But look-alikes such as Epistylis and Tetrahymena exist, and UF/IFAS says a diagnosis needs a compound microscope. A photo and a vet are the reliable route.

Why did the spots come back after they disappeared? Because the visible spot is the protected stage. UF/IFAS and the Merck Veterinary Manual both describe the stage on the fish and the cyst stage as resistant to chemical treatment; only the free-swimming stage is vulnerable. If treatment stops before the label's full course, parasites still on the fish or in cysts restart the cycle.

Does raising the temperature cure ich? The evidence is disputed. Much popular advice recommends it, while aquariumscience.org cites controlled studies that found no benefit. This site does not publish a heat protocol. Ask an aquatic veterinarian, and follow any product label exactly.

Is ich treatment safe for shrimp and snails? Often it is not. Many ich products and salt are harmful to invertebrates or some sensitive fish. Read the label for every animal in the tank, and if it does not say the product is safe for them, ask a vet before using it.

Ich is common and it is manageable, but it rewards patience over speed. Confirm what it is, let the label or a vet set the treatment, and put a quarantine tank between the next new fish and this one.

Here is the case made honestly, including the case against.

What quarantine buys you is not certainty. It is the difference between a problem confined to two fish in a plastic box and a problem loose in the tank you have spent months building. That is the whole argument, and whether it is worth the effort depends on things that are specific to you.

What actually arrives with a new fish

A fish leaving a shop has usually passed through several tanks and several countries, sharing water at each stop with fish from everywhere else. It may have been in that shop's system for days rather than weeks, and shop systems are commonly shared across many tanks.

That journey brings three separate risks, and only one of them is what people mean by disease.

Something the fish is carrying. Parasites and bacteria that a stressed animal in a shared system picks up, and that a healthy-looking fish can carry without showing anything for a week or more.

Something in the water it came in. Whatever is in the bag is in the bag, and pouring it into your tank is a decision you can decline.

The fish itself, unwell from the trip. Shipping, netting, bagging and a change of water chemistry are a lot for a small animal, and the days after arrival are when problems surface.

A quarantine tank addresses all three, mostly by putting time and a physical barrier between the new arrival and everything you already own.

What one actually is

The mental image is the obstacle. People picture a second display tank, and that is not what this is.

A quarantine setup is: a plain container that holds water, a sponge filter, an air pump, a heater, a lid, a thermometer, and somewhere for the fish to hide. No substrate, no plants, no decoration, nothing porous. Bare is deliberate: it is easy to clean, easy to observe, and there is nothing for anything to hide in.

Two points that matter more than the shopping list.

It needs to be cycled, or it needs to be managed like an uncycled tank. A container of fresh water with a fish in it is a tank with no biological filter, and ammonia climbs quickly in a small volume. The standard solution is to keep a spare sponge filter running permanently in your main tank so it is colonized and ready to move across, which is the same principle described in seeding a new filter from an established tank. If you have not done that, you are running a fish-in cycle in a small volume and you need to be testing daily and changing water accordingly, which is a real commitment. Fish-in cycling covers what that involves.

It does not have to be permanently set up. The heater and the sponge filter live in your main tank between uses. The container lives in a cupboard. It goes up the day before you buy fish and comes down four weeks later.

How long, and why nobody agrees

Published recommendations for quarantine length vary widely, and they vary for a defensible reason rather than because someone is wrong. Shorter periods catch the obvious: a fish that arrived already unwell will usually show it within days. Longer periods catch the slower ones, where an organism's life cycle means nothing is visible for a fortnight or more.

The commonly repeated figures sit anywhere from two weeks to a month or more, and you will find confident advocates of each. The rule that survives the disagreement is more useful than picking a number: the clock restarts if anything appears. A fish that shows a symptom on day twelve of a two-week quarantine has not nearly finished. It has started again.

The practical version most keepers land on is a month, because it comfortably covers the slower cases and because a month is a period a person can actually remember.

What you are doing during it

Less than you might think, and none of it involves adding anything.

• Watch, daily. Behavior, breathing, appetite, fins, body surface, waste. You are looking for a change rather than for a specific thing.
• Test the water, daily, because a small volume with no established filter moves fast. How often to test aquarium water covers what to look at.
• Change water when the test says to, with conditioned, temperature-matched water. How to do a water change is the method.
• Feed lightly. A small volume punishes overfeeding, and how much to feed aquarium fish covers the amount.
• Keep separate equipment, or at minimum a separate net, and do not move wet hands from one tank to the other without washing between. Cross-contamination through a shared net defeats the entire exercise.

Do not medicate prophylactically. Treating a healthy fish for a disease it does not have is how resistance develops and how healthy animals get harmed, and this site publishes no medication, no dose and no treatment schedule anywhere. If something appears during quarantine, that is a diagnosis question, and when to contact an aquatic veterinarian and how to find one covers who can actually answer it. The advantage you have bought yourself is that the problem is in a bare box with two fish in it rather than in your display tank.

The honest case against

This site's rule is that no page recommends a purchase the reader does not need, so here is the other side stated properly.

Quarantine is genuinely more work, and done badly it is worse than not doing it. A new fish held in an uncycled container with rising ammonia, unwatched, is being harmed by the quarantine rather than protected by it. A poorly run quarantine is a worse outcome than adding the fish straight to a healthy, stable, uncrowded tank.

It also has a real cost in attention. Daily testing and daily water changes for a month is a commitment, and a person who knows they will not do it is better served by being honest about that now.

Where the argument is weakest: a single species-only tank you never plan to add to again, or a first fish going into a brand new empty tank, where there is nothing to protect.

Where it is strongest, and where it is close to non-negotiable: a tank that already holds fish you would be upset to lose, an expensive or long-lived fish going in, a tank with shrimp or snails in it, or fish bought from a source you have no history with.

If you are not going to do it

The reduced version, which is better than nothing:

• Never pour bag water into your tank. Net the fish across and discard the water.
• Acclimate slowly to temperature and to chemistry.
• Buy from one shop you have a history with, and look at its other tanks before you buy from any of them. A dead fish in a neighboring tank on a shared system is information.
• Do not buy on the day new stock arrives. Give the shop a week to find out what it received.
• Watch your own tank closely for a fortnight afterward, and know what you would do if something appeared. Something is wrong with my tank is the order of checks.

The short version

• A quarantine tank is a bare container, a sponge filter, a heater and a lid. It is not a second display tank.
• Keep a spare sponge filter running in your main tank so it is ready when you need it.
• Published durations disagree. The rule that survives is that the clock restarts if anything appears.
• Watch and test. Add nothing.
• Skip it honestly rather than run it badly, and if you skip it, at least never pour the bag water in.

FAQ

How long should new fish stay in a quarantine tank? Published recommendations range from two weeks to a month or more, and they disagree for defensible reasons. The rule that survives the disagreement is that the clock restarts if anything appears, and the practical version most keepers land on is a month.

Does a quarantine tank need to be cycled? Yes, or it needs to be managed like an uncycled tank, with daily testing and water changes as the results demand. The standard solution is to keep a spare sponge filter running permanently in your main tank so it is colonized and ready to move across.

Should I medicate new fish in quarantine as a precaution? No. Treating a healthy fish for a disease it does not have is how resistance develops and how healthy animals get harmed. If something appears during quarantine, that is a diagnosis question for an aquatic veterinarian, and the advantage you have bought is that the problem is confined to a bare box.

What equipment does a quarantine tank actually need? A plain container that holds water, a sponge filter, an air pump, a heater, a lid, a thermometer and somewhere for the fish to hide. No substrate, no plants, nothing porous, and it does not have to be permanently set up.

What if I decide not to quarantine? The reduced version is better than nothing: never pour bag water into your tank, acclimate slowly, buy from one shop you have a history with, do not buy on the day new stock arrives, and watch your own tank closely for a fortnight afterward.

So the order below is deliberate. Gather before you fix, because the fixing destroys the evidence, and the evidence is what tells you whether the rest of them are next.

Minute one: get the body out

Remove the dead fish now. A body decomposing in a tank adds to the ammonia load, and in a small tank that alone can push a marginal situation into a bad one.

Take it out with a net, not with your hands, and look at it properly under good light before you dispose of it. This is the only chance you get.

Do not flush it. Wrap it and put it in the household waste, or bury it away from any water. Disease and non-native species both travel that way.

Minute two to ten: test the water, before you change anything

This is the step people skip, and it is the one that answers the question.

Test ammonia, nitrite, nitrate, temperature and pH, and write the numbers down with the time. Do it before a water change, because a water change destroys the reading you most need.

If you have a record of previous readings, compare. A tank that has been at zero and zero for weeks and is suddenly not is telling you something specific. Keeping an aquarium water test log covers why the columns most people leave out, what you did and what you saw, are the ones that make a log worth having.

Two things this page will not do. It will not name a level at which fish die, because there is no honest single number: ammonia toxicity rises sharply with pH and with temperature, so the same reading is a different exposure in two tanks. And it will not tell you that your reading is fine. What is reliable is that any detectable ammonia or nitrite in a tank where a fish has just died is the finding, and it is where you start. Ammonia reading above zero and nitrite reading above zero are the pages for each.

If your kit gives you a result that seems impossible, do not dismiss it and do not act on it blindly either. When a test kit gives a wrong reading covers the control test, which is the way to resolve a suspect kit rather than argue with it.

Minute ten to twenty: look at the fish that are still alive

The survivors are better evidence than the body, because you can watch them.

• Where are they in the tank? All crowded at the surface is a specific and urgent signal, covered in why fish gasp at the surface.
• Are they breathing fast? Count gill movements against a normal day if you have any sense of one.
• Are fins clamped? A fish holding its fins tight to its body is a fish under pressure.
• Is anyone hiding who does not normally hide?
• Is anyone else showing marks, spots, ragged fins, cloudiness or swelling?
• Are they eating? Do not feed as a test right now, but note whether they had eaten normally yesterday.

If every remaining fish looks and behaves normally, that shifts the odds toward something specific to the one that died rather than something environmental.

Minute twenty to forty: what changed in the last two weeks

Most sudden single deaths trace to something that happened, and the something is usually recent. Work through this honestly rather than defensively.

• New fish added, even one, even weeks ago. New arrivals bring things in, which is the whole argument for a quarantine tank.
• A filter cleaned, especially if the media was rinsed under a tap. That is one of the more common ways a tank loses its cycle overnight, and an aquarium cycle crash covers what it looks like afterward.
• A large or unusual water change, or a change done without conditioner.
• A power cut, even a short one. A filter that stops for hours is a filter that may come back with dead bacteria in it.
• A temperature swing, a heatwave, a window left open, a heater that failed.
• Anything added to the water: a treatment, a plant fertilizer, a new decoration that was not rinsed.
• Anything used in the room. Aerosols, paint, cleaning sprays, insecticide, scented products. The water surface takes in what is in the air, and this cause is invisible unless you go looking for it.
• New decor or new substrate, particularly anything painted, metallic or of unknown origin.
• Overfeeding, or food left uneaten and decaying under the substrate.

Write down anything on that list that happened. The pattern is often obvious once it is written and invisible while it is in your head.

Minute forty to sixty: act on what you found, in this order

1. If ammonia or nitrite is detectable, do a water change with conditioned, temperature-matched water. How to do a water change is the method. Then test again after a few hours.
2. If the temperature is wrong, bring it back gradually. Fast temperature corrections are their own stress.
3. If you suspect something got into the water, a water change is the response, and repeat it rather than doing one enormous one.
4. If everything reads clean and the other fish are fine, do nothing dramatic. Test again tomorrow and the day after. A single death in a tank that is otherwise healthy happens, and reacting to it with a large intervention causes more harm than it prevents.
5. Do not add any treatment. You do not have a diagnosis, and medicating a tank you have not diagnosed is how a single death becomes several.

The part nobody wants to read

Sometimes the answer is that you will not find out. A fish can arrive from the shop already carrying something. An individual can have an internal problem that was never going to be visible. Fish also have lifespans, and some of the species sold as beginner fish are short-lived.

A single death in a tank with clean water, healthy tankmates and no recent changes usually stays unexplained, and that is a tolerable outcome. What is not tolerable is a second and a third, because that is a pattern and a pattern has a cause.

If a second fish dies within a week, or if several are showing symptoms, stop treating this as a mystery to solve alone. When to contact an aquatic veterinarian and how to find one covers how to find one, what to have ready before you call, and the fact that much of this work is done remotely with photographs and your test results. Multiple deaths with clean water is exactly the situation that page describes as the threshold.

What not to do in the first hour

• Do not do a large water change before testing. You destroy the evidence.
• Do not add medication. No diagnosis, no treatment.
• Do not strip and restart the tank. You would be killing the filter bacteria that are keeping the survivors alive.
• Do not immediately replace the fish. Whatever this was, a new arrival walks into it.
• Do not clean the filter today. It is the one part of the system you most need working.

The short version

• Body out first. Look at it under light before you dispose of it.
• Test before you change anything. Write the numbers and the time down.
• Watch the survivors. They are the better evidence.
• List what changed in two weeks, on paper.
• Correct what the test found, gently, and then wait rather than intervene further.
• One unexplained death is survivable. Two is a pattern and a vet call.

FAQ

What should I do first when I find a dead fish in my tank? Get the body out with a net, look at it properly under good light before you dispose of it, and do not flush it. Wrap it for the household waste or bury it away from any water, because disease and non-native species both travel through flushing.

Should I do a water change straight away? No. Test first. Test ammonia, nitrite, nitrate, temperature and pH and write the numbers down with the time, because a water change destroys the reading you most need.

What ammonia level kills fish? There is no honest single number. Ammonia toxicity rises sharply with pH and with temperature, so the same reading is a different exposure in two tanks. What is reliable is that any detectable ammonia or nitrite in a tank where a fish has just died is the finding, and it is where you start.

What if the water tests clean and the other fish look fine? Do nothing dramatic. Test again tomorrow and the day after. A single death in a tank with clean water, healthy tankmates and no recent changes often stays unexplained, and reacting to it with a large intervention causes more harm than it prevents.

When does a fish death become a vet question? If a second fish dies within a week, or several fish are showing symptoms, stop treating it as a mystery to solve alone and contact an aquatic veterinarian. Much of that work is done remotely with photographs and your test results.

The internet will hand you a single label for it, and that label is the problem. "Swim bladder disease" is not a diagnosis. It is a description of what you are looking at, and the things that produce it range from a meal that was too large to an injury to a bacterial infection to poor water. They do not respond to the same thing.

What this page can do is tell you what the pattern points at and what to check, in what order.

The three patterns, and what each suggests

Watch for two or three minutes and note which one you have.

Floating, and unable to get down. The fish is at or near the surface, often tail-up, and swimming downward takes visible effort that it loses the moment it stops. Most commonly associated with gas or with distension inside the body cavity: a large meal, food that swelled after being eaten, constipation, or trapped air.

Sinking, and unable to get up. The fish rests on the substrate and rises only with continuous effort. Less commonly discussed and generally more serious, because the everyday causes of positive buoyancy do not explain it. Infection, organ problems and injury are more likely here.

Rolling or listing sideways. The fish cannot hold its axis and tips or rolls, sometimes intermittently. This one is the least specific of the three and is seen with infection, with neurological problems, and in the later stages of several unrelated illnesses.

Any of the three appearing suddenly in several fish at once is not a buoyancy problem at all. It is a water problem, and it needs the water tested immediately rather than the fish examined.

Check the water first, every time

This is not a formality. It is the step that most often changes what you do next.

Test ammonia, nitrite, nitrate and temperature and write the numbers down. Aquarium water parameters explains what each one is telling you and how often to test aquarium water covers the routine that would have caught it earlier.

Two things this site will not do here. It will not give you a figure at which fish begin to lose buoyancy control, because no honest single figure exists: ammonia toxicity depends on pH and temperature, so the same reading is a different exposure in two different tanks. And it will not tell you that a particular reading is fine. What is reliable is simpler: detectable ammonia or nitrite in a tank with an unwell fish is a finding, and correcting it comes before anything else you might try. Ammonia reading above zero is where to go if that is what you found.

Temperature deserves its own look. A fish that is much colder than its species is comfortable at digests slowly, and slow digestion is behind a large share of the floating cases. A tank that has drifted cold, or that swings several degrees between day and night, is worth correcting on its own account, and aquarium water temperature covers what stable actually means.

Then look at the feeding

If the water is clean and one fish is floating, feeding is the next place to look, and it is the most commonly involved factor in the everyday version of this.

The things that come up repeatedly:

• Too much food, too fast. A fish that gulps a large meal takes in air with it and swallows more than the gut can move.
• Dry food that expands. Flakes and pellets absorb water. Food that swells after it is eaten rather than before is a mechanical problem inside a small animal.
• Food taken at the surface by a fish that gulps air along with it.
• A diet that is entirely dry, with nothing that moves through the gut easily.
• Species that are built for this. Fancy goldfish and some short-bodied, round-bodied varieties have compressed internal anatomy as a consequence of how they were bred, and buoyancy trouble is much more common in them. That is a conformation issue, not something you did.

The general corrections are unglamorous and they are within your control: feed less at a time, soak dry food before it goes in so it swells in the water rather than in the fish, and feed a varied diet rather than one dry product. How much to feed aquarium fish covers the amount, which is the part almost everybody gets wrong in the same direction.

You will find pages recommending a specific fasting period followed by a specific vegetable in a specific quantity. This site does not publish that protocol, for reasons set out below.

Supportive care, which is not treatment

While you work out the cause, there are things that reduce harm and none of them involve adding anything to the water.

• Lower the water level slightly in a tank where a sinking fish is struggling to reach the surface, or make sure a floating fish is not stranded in air. A fish held out of water dries and damages.
• Reduce the flow. A fish that cannot control its position is being pushed around by a current it would normally handle.
• Give it somewhere to rest, low and soft. A broad plant leaf or a smooth surface near the bottom.
• Keep the light low and the room quiet. A stressed fish that cannot swim away from disturbance is in a worse position than a healthy one.
• Do not feed for now. If digestion is implicated, adding more is the wrong direction. This is a pause while you investigate, not a treatment schedule.
• Keep the water clean and stable. Steady conditions are the environment recovery happens in, and how to do a water change is the method.

What this site does not publish, and why

No medication, no dose, no fasting duration presented as a cure, no salt concentration, no temperature protocol.

The reason is specific to this symptom rather than general. Buoyancy loss has causes that look identical from outside and need opposite responses. A fish that floated after a large meal and a fish that is listing because of an internal infection present in a very similar way to an owner watching through glass. A protocol aimed at the first does nothing for the second while days pass. Worse, several of the treatments circulated for this are dosed by tank volume, are harmful to invertebrates, and damage the filter bacteria the tank depends on, which turns one unwell fish into a tank with an ammonia problem.

There is also a hard limit on what anyone can determine from a description. Distinguishing gas from an organ problem from an injury is a physical examination, and in some cases an imaging question. That is not something a page can do, and a page that pretends otherwise is selling confidence rather than information.

A fish that is sinking rather than floating, that is listing without an obvious feeding trigger, that has been like this for more than a couple of days, or that is one of several affected, is a veterinary question. When to contact an aquatic veterinarian and how to find one covers how to find one, what to have ready, and the fact that a lot of this work is done by phone or video because the patient travels badly. It also covers the humane end of the conversation, which is worth knowing about before you need it.

The short version

• The pattern matters. Floating, sinking and rolling suggest different things.
• Several fish at once means the water, not the fish.
• Test first. Correct what the test finds before anything else.
• Feeding is the most common everyday factor in floating, and the corrections are all about less and slower.
• Supportive care is flow, rest and clean water. It is not medication.
• Sinking, listing, or no improvement in a few days is a vet call.

FAQ

Is swim bladder disease an actual diagnosis? No. It is a description of what you are looking at, and the things that produce it range from a meal that was too large to an injury, a bacterial infection or poor water. They do not respond to the same thing, which is why a single label leads people to the wrong response.

What does it mean if several fish lose balance at once? That is not a buoyancy problem at all. It is a water problem, and it needs the water tested immediately rather than the fish examined.

Why is feeding the first suspect when one fish is floating and the water is clean? Because it is the most commonly involved factor in the everyday version of this: too much food too fast, dry food that swells after being eaten, air gulped at the surface, and a diet that is entirely dry. The corrections are to feed less at a time, soak dry food first, and vary the diet.

What can I do for the fish while I work out the cause? Supportive care, none of which involves adding anything to the water: reduce the flow, give the fish somewhere low and soft to rest, keep the light low and the room quiet, pause feeding, and keep the water clean and stable.

When is a floating or sinking fish a vet call? A fish that is sinking rather than floating, that is listing without an obvious feeding trigger, that has been like this for more than a couple of days, or that is one of several affected is a veterinary question. Much of that work is done by phone or video because the patient travels badly.

That answer is wrong often enough to matter, because at least three different things produce a damaged fin and only one of them is an infection. Treating the tank for a disease it does not have means medicating healthy fish, and often means harming the filter bacteria as well, while the actual cause carries on doing what it was doing.

So the useful question is not "what medicine", it is "what is the shape of the damage, and what else is in the tank".

Look at the edge, not the amount

How much fin is missing tells you very little. What the missing edge looks like tells you a great deal.

Two more observations sharpen it further.

Is it one fish or several? Damage confined to one long-finned individual in a mixed tank points hard at nipping or at decor. Damage appearing across several fish at once points at the water.

Is it getting worse day to day? Take a photograph today and another in two days from the same angle. A tear that is unchanged, or that is visibly rebuilding a slightly ragged new edge, is healing. A margin that is creeping steadily toward the body is not.

Cause one: something in the tank is sharp

This is the most under-diagnosed of the three and the easiest to fix.

Run a check on everything hard in the tank. Plastic plants with molded seams. Rough or holed decorative ornaments. Broken slate. Anything the fish squeezes through or hides inside. Filter intakes that a long fin can be pulled against.

The classic test is a length of pantyhose or a fine cloth drawn over the surface. If it snags, a fin snags.

Long-finned fish and fish that startle easily are at much higher risk, because the damage happens in a fraction of a second when something spooks them at night.

Cause two: a tankmate is biting

Nipping is behavior, so you diagnose it by watching rather than by inspecting.

Watch the tank for ten quiet minutes with the room lights low and yourself out of view, twice, at different times of day. Feeding time and the hour before lights-out are when it usually happens.

Signs that point this way:

• One fish is targeted repeatedly and the others are untouched.
• The damage is on the trailing edges, in bites rather than in a receding margin.
• The victim has the longest or most colorful fins in the tank.
• You can see the chasing. Nipping is not usually subtle once you are looking for it.

Fixing it is a stocking question, not a medicine question. A group that is too small, a tank that is too crowded, a species combination that was never going to work, or too little cover for the fish being harassed. The tankmate compatibility question is a subject in its own right and it deserves more room than this page has.

Cause three: water quality, which is what fin rot really is

The infection people call fin rot is usually opportunistic. The bacteria involved are ordinary residents of an ordinary tank, and a healthy fish's immune system handles them. What lets them get a foothold is a fish that is already under pressure, and the most common source of that pressure by a wide margin is the water.

So before anything else, test it. Ammonia, nitrite, nitrate, temperature, and note what the readings actually are rather than whether they feel acceptable. If you are unsure what to test or how often, how often to test aquarium water covers the routine, and aquarium water parameters covers what each reading is telling you.

This site does not publish a threshold at which fin damage begins, and that is deliberate. The toxicity of a given ammonia reading depends on pH and temperature, so the same number on two tanks is not the same dose to two fish. What is reliable is the direction: any detectable ammonia or nitrite in a tank with a damaged fish is a finding, and it is the first thing to correct, whatever the number is. Ammonia reading above zero is the page for that.

Other pressures that stack on top of water quality: a temperature that swings, a tank that never gets a water change, overfeeding and the decay that follows, and a recent move or a recent addition of new fish.

What to do first, in order

1. Test the water. Ammonia, nitrite, nitrate, temperature. Write down what you get.
2. Correct anything the test found, starting with a water change if ammonia or nitrite is detectable. How to do a water change is the method.
3. Find and remove anything sharp. The cloth test.
4. Watch the tank twice for ten minutes with yourself out of sight, and note who chases whom.
5. Photograph the fin from the same angle each day so you have evidence rather than an impression.
6. Keep the routine steady. Clean, stable water is the condition a fin heals in. Fins regrow, though the color and the shape of the regrown tissue may not match what was there.

If the water is clean, nothing is sharp, nobody is biting, and the margin is still receding, that is the point at which this stops being something to solve from a web page.

The treatment this site does not publish, and why

You will not find a medication name, a dose, a salt concentration or a treatment schedule here, and not because the information is hard to find. It is because publishing it would be irresponsible in a way that is easy to miss.

Aquarium medications are dosed by water volume, and a dose is only as accurate as the volume estimate behind it, which for most tanks is the number printed on the box rather than the actual water in the glass. The same compound that is routine for one species is dangerous to another. Several common treatments are lethal to shrimp and snails, and several harm the filter bacteria the whole tank depends on, which converts a fin problem into an ammonia problem within a week.

Most importantly, the fin is a symptom. If the cause is a sharp ornament or a nipping tankmate, the medication treats nothing at all while the damage continues.

When the water is clean and the fish is still declining, or when there is an ulcer, a growth, a wound that is not healing, or fungus-like tufting, that is a veterinary question. When to contact an aquatic veterinarian and how to find one covers who to call, how to find one near you, and what to have ready before you do. A great deal of fish veterinary work is done remotely, so distance is less of an obstacle than people assume.

The short version

• The shape of the edge tells you the cause. A clean tear is injury, a receding margin is infection, crescent bites are a tankmate.
• One fish means decor or a tankmate. Several fish means the water.
• Test first, always. Correct what the test finds before you consider anything else.
• Fins regrow in clean, stable water.
• No page can tell you what to medicate for. A vet can.

FAQ

How can I tell fin rot from fin nipping? Look at the edge, not the amount. Fraying that starts at the outer edge and works inward, often with a pale, cloudy or discolored margin, points at an infection process. Small crescent bites out of the trailing edge, usually on one fish more than others, point at a tankmate. A clean, straight tear with otherwise healthy edges points at physical damage from something sharp.

Do damaged fins grow back? Yes. Clean, stable water is the condition a fin heals in, though the color and the shape of the regrown tissue may not match what was there before.

Should I medicate for fin rot straight away? No. Test the water first and correct anything the test finds, check the tank for anything sharp, and watch for nipping. Medications are dosed by water volume, several are lethal to shrimp and snails or harm the filter bacteria, and if the cause is an ornament or a tankmate the medication treats nothing while the damage continues.

Why does damage on several fish at once point at the water? Because the causes that target one fish are individual: a tankmate nips a specific victim, and decor injures the fish that swims through it. Damage appearing across several fish at once points at something they all share, which is the water.

When is a damaged fin a question for a vet? When the water is clean, nothing is sharp, nobody is biting and the margin is still receding, or when there is an ulcer, a growth, a wound that is not healing, or fungus-like tufting. An aquatic veterinarian can diagnose what a page cannot, and a great deal of that work is done remotely.

How to Identify It

Green spot algae, often written GSA, appears as small, distinct, bright green circular spots. Two properties separate it from everything else green in your tank:

• It is hard. A magnet cleaner glides over it. A cloth does nothing. You feel it as a raised dot under your fingertip.
• It stays put and stays discrete. It forms spots rather than a film, and the spots hold their shape as they grow.

Compare that with the alternatives: a soft green film that wipes away easily is a different, much less stubborn green algae; dark bristly tufts on leaf edges and hardscape are black beard algae; a green tint through the water itself, with the glass clean, is green water, which is free-floating and needs an entirely different approach; and a brown dust in a young tank is a diatom phase that passes on its own.

Green spot algae also appears on the leaves of slow-growing plants, especially anubias, and on hardscape. On the glass it is a nuisance. On anubias leaves it is close to permanent.

What Causes It

Planted-tank references, including 2Hr Aquarist and Aquasabi, converge on a short list: strong or long lighting, insufficient CO2 relative to the light provided, and low phosphate. Some sources add that very high phosphate can also be implicated, which is a useful reminder that "balance" here is genuinely a balance rather than a direction.

The low-phosphate association is the one worth understanding, because it inverts the usual instinct.

Most algae problems in a beginner tank are excess problems: too much food, too much waste, too much light. So when algae appears, the reflex is to feed less, change more water and dose nothing. With green spot algae, that reflex tends to backfire. If phosphate is the limiting nutrient, driving it lower does not starve the algae. It starves the plants, which stops them competing, and the algae takes the light that the plants are no longer using.

That is why green spot algae is common in two very different tanks: heavily lit planted tanks running lean on fertilizer, and lightly stocked, lightly fed tanks with a bright light on for a long day.

What Actually Reduces It

Shorten the photoperiod. Light duration is the easiest variable to change and the one most directly under your control. I will not name a number of hours, because the correct photoperiod depends on your fixture's intensity, your plant load and your tank, and any figure quoted here would be invented rather than measured. Reduce it, watch for a few weeks, and judge by the result.

Stop the tank from running nutrient-starved. In a planted tank this means a fertilizer that actually contains phosphate, used according to its label. In an unplanted, lightly stocked tank it often means that the tank is simply very clean and very brightly lit, and the practical response is less light rather than deliberately dirtying the water. If your nitrate consistently reads at or near zero in a stocked tank, that is worth understanding on its own terms, and aquarium nitrate levels covers what a low reading does and does not mean.

Improve circulation and, in an injected tank, CO2 consistency. Sources associate green spot algae with CO2 that is insufficient for the amount of light given. In a low-tech tank without injection, the practical equivalent is reducing the light rather than raising anything.

Clean the glass on a routine, before it hardens. Young spots come off far more easily than established ones. Making the front pane part of the weekly routine is less work than a quarterly battle.

Accept it on anubias. Green spot algae on a slow-growing leaf is very difficult to remove without damaging the leaf. Removing badly affected old leaves is usually the better outcome, and the plant will replace them.

Removing It Without Damaging the Tank

This is the practical section, and it contains the one genuine risk on this page.

On glass: a blade scraper is the tool that works. Algae scraper blades, or a plain razor blade in a holder, cut hardened spots off cleanly. Keep the blade flat to the pane and work in overlapping passes. Watch two things: keep sand and gravel grains off the blade, since a trapped grain will scratch the glass, and stay away from the silicone seams, because a blade run along a seam can cut the sealant and that is a leak you have created yourself.

On acrylic: do not use a blade. Acrylic scratches easily and permanently, and a razor blade will ruin the panel in a single pass. Use a pad or scraper explicitly sold as acrylic-safe, expect it to take more patience, and accept that some hardened spots may need repeated gentle passes rather than force. If you are not certain whether your tank is glass or acrylic, tap it, look at the corners (acrylic tanks typically have seamless, rounded or bonded corners rather than silicone joints), and if in doubt treat it as acrylic.

Magnet cleaners are for maintenance between scrapes. They do not remove hardened green spot algae and running one over grit is another way to scratch a panel.

Nerite snails are widely reported as the most effective grazers on green spot algae, and they are hardy enough to be a fair addition to a stable tank. The usual caution applies: add an animal because you want to keep it, not as a cleaning appliance, and check that its needs match your tank first. No snail will fully clear a heavily affected pane.

When Green Spot Algae Is Telling You Something

Most of the time this algae is cosmetic. Two patterns are worth reading as signals:

• It appears fast and everywhere, including on new plant growth. That points at a light level the plants cannot keep up with, and it is worth reducing the photoperiod rather than scraping more often.
• It arrives alongside other algae types at the same time. Multiple algae appearing together usually means the tank has changed rather than that you have several separate problems. Look for what changed: a new light, a longer timer setting, a rescape, a change in stocking or feeding, or the tank having moved into direct sunlight.

Algae does not make fish ill. If your fish are unwell as well as your glass being spotted, treat those as two separate matters, and when to contact an aquatic veterinarian covers where the line falls.

FAQ

Why do I get green spot algae on the glass when my tank is clean? Because it is generally not a cleanliness problem. It is associated with strong or long lighting combined with a shortage of nutrients, particularly phosphate, which is a common combination in a bright, lightly stocked tank.

Will feeding less get rid of green spot algae? Usually not, and it can make things worse. Feeding less lowers the nutrients your plants need to compete. Reducing light duration is the more reliable lever.

How do I get green spot algae off acrylic? Never with a blade. Use a pad or scraper sold as acrylic-safe, and accept that it takes longer. A razor blade permanently scratches acrylic.

Does green spot algae hurt plants? It can. On slow-growing leaves such as anubias it blocks light and does not come off cleanly, so removing badly affected older leaves is often the better option.

Do snails eat green spot algae? Nerite snails are the most commonly recommended and are reported to graze it effectively. They will not fully clear a badly affected pane, and they should be added because you want to keep them, not purely as a cleanup measure.

How to Know It Is Black Beard Algae

Black beard algae, usually shortened to BBA, is a red algae despite its color. In an aquarium it appears as short dark tufts, from near-black to dark green to grayish, with a bristly texture. It attaches hard. That is the identifying feature: green algae wipe or scrape off, and BBA holds on so tightly that you usually end up removing the leaf rather than the algae.

Where it appears is diagnostic, and this is the part most guides leave out.

Read that table backward and it becomes a diagnosis. BBA is a settler. It wins on surfaces that do not renew themselves, in water that carries plenty of organic material past them.

What Actually Causes It

Aquascaping sources are consistent on the main drivers, and they disagree less than the forums suggest. Across 2Hr Aquarist, BucePlant and other planted-tank references, the recurring list is: unstable or insufficient CO2 in injected tanks, accumulated organic waste, too much light for the plant load, and general instability.

Instability, not any single number. The most commonly named trigger in CO2-injected tanks is fluctuating CO2 rather than a specific level. Plants that cannot photosynthesize reliably stop competing, and algae takes the surplus.

Organic waste. Detritus in the substrate, a filter that has not been serviced in a long time, decaying leaves and uneaten food all raise the organic load. This is the driver that applies to every tank, whether or not it injects CO2.

Light. Too much light for the amount of plant growth the tank can support. Note that this is a ratio, not a threshold. I am not going to give you a photoperiod in hours, because the right answer depends on your fixture, your plants and your tank, and a number invented here would be worthless.

Flow. BBA thrives in high-flow spots, but low flow causes its own problems by letting debris settle. What you want is even circulation without blasting a specific area.

The Low-Tech Reality Check

Here is the honest part that most BBA guides skip. Most of the standard advice assumes a CO2-injected planted tank, because that is the kind of tank the people writing it keep. "Stabilize your CO2" is useless advice if you do not inject CO2.

If you run a low-tech tank, which describes most beginner setups, this is what is left after you subtract the CO2 advice:

• Reduce the organic load. Clean the filter on a sensible schedule, siphon the substrate surface where debris collects, remove dying leaves, and feed less. This is the single most actionable lever in a low-tech tank.
• Keep water changes regular. Consistency is doing more work here than volume. How to do a water change covers the mechanics.
• Shorten the photoperiod rather than dim the light. Fewer lit hours is the easier variable to change and it directly reduces the energy available to algae.
• Prune and discard. Cut off badly affected leaves and throw them away rather than trying to clean them. On anubias and java fern, an affected leaf rarely recovers its appearance.
• Treat hardscape separately. Rocks and wood can be removed, scrubbed, and in some cases left to dry out entirely before going back in. This is far easier than treating them in the tank.
• Accept some of it. A little BBA on a piece of driftwood in an otherwise healthy tank is a cosmetic issue. Chasing it to zero causes more damage than living with it.

If your tank is planted and still establishing, cycling a planted aquarium covers the early instability that gives algae its opening in the first place.

About the Chemical Treatments

There are three categories people reach for: glutaraldehyde-based "liquid carbon" products, hydrogen peroxide used as a spot treatment, and general algaecides. They do work on black beard algae. Treated tufts typically turn red or pink and then break down.

This page does not publish a dosing rate, a volume per unit of water, or a spot-treatment concentration for any of them, and here is why.

These products are toxic by design. The margin between the amount that kills algae and the amount that harms the tank's inhabitants is narrower than the confident tone of most forum posts suggests, and it is not the same for every animal in your tank. Shrimp and other invertebrates commonly die at amounts that the fish in the same tank tolerate. Some plants, vallisneria among the most frequently reported, react badly to glutaraldehyde-based products even when used as directed. Sensitive fish species and fry are also more vulnerable than robust adult community fish.

So:

• The manufacturer's label is the only authority on how much to use. Not this page, not a forum post, not a video. Read it before the bottle is anywhere near the tank.
• If you keep shrimp, snails or any invertebrate, read the label specifically for what it says about invertebrates, and if it does not say, assume the answer is bad.
• "Double it for stubborn algae" is how people kill their livestock. It is one of the most repeated pieces of advice in this hobby and one of the worst. The algae being stubborn is not evidence that a higher amount is safe.
• Spot treating means removing the affected item where possible, rather than adding chemicals to a tank full of animals. Taking the rock out is safer than treating around the fish.
• Never combine treatments or follow one product with another without knowing what happens when they meet. You will not be able to interpret the result if something goes wrong.

None of the above is a reason to feel helpless. The non-chemical measures in the previous section are the ones that change the tank's trajectory. Chemicals remove the algae that is already there while leaving the conditions that grew it intact, which is why so many people report it coming straight back.

Algae itself does not make fish sick. If your fish are unwell rather than your glass being ugly, that is a separate matter and when to contact an aquatic veterinarian covers it.

Biological Control

Siamese algae eaters (Crossocheilus species) are the fish most often credited with eating BBA, and reports of their effectiveness range from enthusiastic to disappointed. Two honest caveats: they are frequently misidentified in shops, with several similar-looking species sold under the same name, and they grow larger and more active than beginners expect, which makes them a poor fit for a small tank.

The wider principle applies here as it does elsewhere on this site: an animal is not a cleaning product. If a fish does not suit your tank on its own merits, do not buy it to solve an algae problem.

If It Is Not Black Beard Algae

Hard green dots on the glass that resist wiping are a different organism with a different cause: see green spot algae on the glass. A brown dusty film in a new tank is a diatom phase that passes on its own: see brown algae in a new aquarium.

FAQ

Will black beard algae go away on its own? Generally not. Unlike the diatom phase in a new tank, BBA persists until the conditions that favor it change. It also rarely explodes overnight, which means you usually have time to fix causes rather than reach for a bottle.

Does black beard algae harm fish? No. It harms plants indirectly by covering leaves and blocking light, and it is unattractive, but it is not toxic to fish or invertebrates.

Can I use hydrogen peroxide or liquid carbon on it? People do, and it works. This site does not publish quantities for either, because overdosing kills shrimp and can damage sensitive plants, and the correct amount is what the manufacturer's label says for that specific product. Read the label, and treat removed hardscape outside the tank where you can.

Why does it grow on my filter outlet? Because that is a fixed surface with a constant supply of nutrient-carrying water moving past it. Location is a clue: BBA colonizes surfaces that do not renew themselves.

I do not inject CO2. Does the CO2 advice apply to me? No, and that is worth knowing, because most BBA guides are written for injected tanks. In a low-tech tank the levers are organic waste, light duration, flow, regular water changes and physical removal.

First, Confirm It Is Diatoms

Three brown-ish things turn up in tanks and they are not the same. Get this right before you act.

If you can wipe it off the glass with a finger and it clouds the water as it goes, you have diatoms.

Why a New Tank Gets Them

Diatoms are single-celled algae that build glass-like shells out of silica, so they need dissolved silicates in the water. A new aquarium supplies those generously: fresh substrate leaches silicates, tap water brings more in with every top-up and water change, and new sand or gravel is often the biggest single source.

The second half of the explanation is the more useful one. A new tank has almost no competition. The bacteria, the plants and the other microorganisms that will eventually take up the same nutrients are not established yet. Diatoms tolerate low light and unstable conditions better than most green algae, so they get there first.

The 2Hr Aquarist and Aquasabi both describe brown films appearing within the first weeks of a new setup, and Aquarium Science describes them fading as the tank matures. The specific timelines quoted across otherwise credible sources vary widely, from a few weeks to several months, which is itself informative: nobody can promise you a date, because it depends on your substrate, your source water and how quickly the rest of the system establishes.

What I can tell you plainly is the direction. Diatoms peak and then decline as the tank matures. They are usually the algae that a keeper never sees again.

What Actually Shortens It

Ranked honestly, from what matters most to what mostly sells product.

1. Time and a maturing tank. This is the real answer and it is unsatisfying. Every week the biological filter, the plants and the competing microorganisms take more of the available nutrients, and the diatoms lose ground.

2. Physical removal, repeatedly. Wipe the glass before each water change, siphon the substrate surface where the film has settled, and gently rinse or wipe affected decor. This does not stop them, but it keeps the tank looking acceptable while the phase passes, and removing biomass removes some of the silicate that would be recycled.

3. Plants. Fast-growing plants compete for nutrients and speed up the transition. In a planted setup, diatoms usually pass faster.

4. Water changes with a lower-silicate source. If your tap water is high in silicate, changing water keeps topping up the supply. RO or a silicate-absorbing resin will reduce it. This is a real intervention, and it is also a significant expense and complication for a phase that ends by itself. Reasonable for a planted display tank, overkill for a first community tank.

5. Chemical algaecides. Not recommended here, and not necessary for a self-limiting phase. This site does not publish dosing for anything added to a tank with livestock in it.

The Cleanup Crew Mistake

The most common piece of advice on this topic is to buy otocinclus catfish or nerite snails, because both eat diatoms. Both do eat diatoms. That advice still deserves a warning attached.

Otocinclus are widely reported as fragile, especially in tanks that are new or not fully cycled, and they are usually wild-caught and already stressed by the time they reach a shop. Adding them to a tank that is still cycling is asking a delicate fish to survive ammonia and nitrite in order to solve a cosmetic problem that would resolve on its own. They also starve once the diatoms are gone if nothing replaces that food.

"Your fish come first" has to mean something concrete, and this is one of the places it does: do not buy an animal as a cleaning product for a tank that is not ready for it. If you want otocinclus, get them because you want them, and add them to a finished, stable, mature tank with a food plan. Nerite snails are considerably hardier and a more reasonable choice, though the same logic about a stable tank applies. If a fish you have added is unwell rather than simply stressed, when to contact an aquatic veterinarian covers the boundary.

Brown Film in an Established Tank Is a Different Question

Everything above is about the new-tank diatom phase. If a mature tank that has been clean for months develops a brown film, do not assume it is the same thing.

Look at three possibilities in order:

• It is cyanobacteria, not diatoms. Use the peel test: a mat that lifts off in sheets and smells is not a diatom film.
• Something changed in the source water or the hardscape. New sand, new rock, a filter change, or a different water supply can reintroduce silicates.
• Something in the tank is declining. Low flow, dying plants or reduced maintenance all shift the nutrient balance in favor of whatever grows easiest.

A returning brown film in a settled tank is worth investigating. A brown film in a six-week-old tank is worth wiping.

What Comes After the Diatoms

The usual sequence in a new tank is diatoms first, then green algae as the tank matures and conditions improve. That is progress rather than a new failure. Green spots on the glass tend to arrive later, and are covered in green spot algae on the glass. Dark tufts on plant edges and hardscape are a different beast with a different cause: see black beard algae.

And if the water itself is hazy rather than the surfaces being coated, that is not an algae question at all. Cloudy water in a brand new tank covers the bacterial bloom that often runs alongside this phase.

FAQ

How long does brown algae last in a new tank? Sources vary from a few weeks to several months, and none of them can be specific because it depends on your substrate, your source water and how fast the tank matures. The reliable part is the direction: it peaks and then declines.

Is brown algae harmful to fish? No. It is unsightly and it can smother plant leaves by blocking light, but it does not poison fish or invertebrates.

Does brown algae mean my tap water is bad? No. Silicates in tap water are normal and are not a contamination problem. Diatoms in a new tank are a stage of the tank settling in, not evidence of a water quality failure.

Will more light get rid of brown algae? Increasing light is a common suggestion because diatoms tolerate low light well, but adding light to an immature tank invites green algae instead. Growing plants successfully is the useful version of this idea; simply turning the light up longer is not.

Should I buy otocinclus to clean it up? Not for a new tank. They are delicate, frequently wild-caught, and often die in tanks that are still cycling. If you want them, add them to a mature, stable tank and plan how they will be fed once the diatoms are gone.

What Green Water Actually Is

Free-floating algae are microscopic, they reproduce in open water, and there can be an enormous number of them in a small volume. That is why the tank looks like weak pea soup rather than like something growing on the glass.

Two consequences follow, and they govern everything else on this page:

1. You cannot remove it by cleaning surfaces. There is nothing on the surfaces. The algae is in the water you are looking through.
2. Ordinary mechanical filtration mostly does not catch it. The cells are far smaller than the pore size of standard filter floss and sponge. This is why a perfectly good filter runs all day and the water stays green.

That makes green water fundamentally different from the attached algae most guides lump it in with. Scraping works on green spot algae on the glass because that algae is stuck to something. There is nothing to scrape here.

Why It Started

Green water needs the same two things every algae needs: light and nutrients. A bloom means both were available in excess of what the plants and the rest of the system could take up.

Light is usually the dominant factor, and direct or reflected sunlight on the tank is the single most common trigger I see reported. A tank near a window can receive far more light than any aquarium fixture provides, at intensities plants cannot compete with. A long photoperiod on an aquarium light does the same thing more slowly.

Nutrients come from the usual sources: fish waste, uneaten food, decomposing plant matter, and disturbed substrate releasing what has been stored in it. Blooms often follow an event that released a lot of nutrients at once, such as a rescape, a deep substrate clean, or an ammonia spike in an immature tank. A tank that runs persistently high on nitrate is a tank with the fuel in place, and aquarium nitrate levels covers keeping that in a sensible range.

Instability matters too. New tanks bloom more readily than mature ones because the competing biology (plants, established bacteria, other microorganisms) has not built up yet.

Why Water Changes Do Not Fix It

This is the honest part, and it is the part people find hardest to accept because water changes fix so much else.

A water change removes a proportion of the algae cells and a proportion of the dissolved nutrients. What it leaves behind is a smaller population in clearer water, with the same light and enough nutrients to regrow. Clearer water also means light penetrates better. Blooms frequently come back stronger within days.

This does not mean water changes are wrong. If nitrate is high, or if something died, a water change is correct for those reasons and it removes fuel. Just do not expect the water to stay clear because of it, and do the change properly with temperature-matched, dechlorinated water: how to do a water change covers the mechanics.

What Actually Clears It

UV sterilizer

A UV sterilizer passes water through a chamber where ultraviolet light damages the DNA of free-floating organisms so they cannot reproduce. Because green water algae is free-floating by definition, this is close to a purpose-built solution, and it is the method most commonly recommended by retailers and experienced keepers alike. The dead cells clump and get caught by the filter, so the water clears.

Practical notes rather than promises:

• Reports of how quickly it clears vary from a few days upward depending on the unit, the flow rate and the severity of the bloom. I am not going to quote a fixed number of days, because the honest answer is that it depends on all three.
• Flow rate matters. Water has to spend enough time in the chamber. Plumbing it on the output of a canister filter is the usual arrangement.
• UV bulbs lose output long before they stop lighting up, so an old unit that "does not work" often just needs a bulb.
• A UV unit treats the symptom permanently and the cause not at all. If the tank still sits in direct sun, the bloom returns when the unit is off.

Blackout

Denying the algae light kills it. Aquarium Co-Op describes the blackout approach as doing a large water change, then covering the tank completely so that no light gets in for a period of days (their description is seven to ten), then doing another large water change afterward.

The trade-offs are real and worth stating plainly:

• Plants suffer. Most aquarium plants survive several days of darkness, but some melt, and a heavily planted tank pays a price.
• Dying algae releases nutrients. A large mass of algae dying at once can produce an ammonia spike, which is dangerous to fish and can feed the next bloom. That is why the water change afterward is not optional.
• Complete means complete. Partial darkness gives a partial result. Any light leak keeps the bloom going.
• Fish are fine in the dark for that period. They are not fine in an ammonia spike.

Fine mechanical filtration

A diatom filter or a very fine micron cartridge can physically strip the cells out. It works, it is fast, and it is equipment most beginners do not own. Worth knowing about, rarely worth buying for one bloom.

Nutrient competition

Fast-growing and floating plants compete with algae for the same nutrients and shade the water at the same time. This is a slow, preventative measure rather than a cure for an active bloom, and it works best as part of how the tank is run rather than as a response.

Reducing light

Move the tank out of direct sunlight if you can, or block the window side. Shorten the photoperiod on the aquarium light. I will not give you a target number of hours, because the right photoperiod depends on your fixture's intensity, your plant load and your tank, and any specific figure I quoted would be a guess dressed up as a rule. The principle is sound even when the number is not knowable from here: less light for less time is less fuel.

What Does Not Work

• Scrubbing and scraping. Nothing is attached.
• Filter floss alone. The cells are too small.
• Clarifiers. They clump particles; they do not stop algae reproducing.
• Doing nothing. Unlike a bacterial bloom in a new tank, which starves itself out, green water has a renewable food supply as long as the light is on. It can persist for a very long time.

That last contrast is worth holding on to. Milky white haze in a new tank clears itself: see cloudy water in a brand new tank. Green haze does not.

Is Green Water Harmful?

Not to healthy fish. It is cultured deliberately as a first food for fry, which tells you something about its toxicity. The genuine risks around it are secondary: a very dense bloom can swing oxygen levels between day and night, and a large algae die-off can spike ammonia. Fish that look ill during a bloom are usually reacting to those, or to something unrelated. Algae does not cause disease, and if your fish have visible symptoms rather than stress behavior, when to contact an aquatic veterinarian is the honest next step.

FAQ

Will a water change clear green water? Temporarily at best. It removes cells and nutrients but leaves the light and enough population to regrow, often within days. Do water changes for your test readings, not for the color.

Is green water dangerous to my fish? Not in itself. It is used as fry food. Watch for oxygen swings in a very dense bloom, and for an ammonia spike if a large amount of algae dies at once.

Does a UV sterilizer fix the cause? No. It removes free-floating algae extremely effectively and does nothing about the light and nutrients that produced the bloom. Treat it as a tool, not a solution.

Can I just wait it out? Usually not. A bacterial bloom starves itself out; green water has light and nutrients renewing continuously, so it can persist indefinitely.

My tank is next to a window. Does that matter? Very likely yes. Direct sunlight is the most commonly reported trigger, and it delivers far more light than any aquarium fixture. Blocking it or moving the tank is the cheapest intervention available.

Step One: Is It a Bloom or Is It Debris?

These two look similar through the front glass and have different causes, so separate them before you do anything.

Take a glass of tank water and hold it against a sheet of white paper, then leave it on the counter for an hour.

• The glass looks clear on its own, and the sediment settles to the bottom: you have suspended particulate. Something physical is in the water column: stirred substrate, detritus knocked loose, fine sand, or dust from new decor or a new filter medium.
• The glass still looks milky after an hour, with nothing settled: you have a bacterial bloom. Free-floating bacteria do not settle out, which is exactly why they are so persistent.
• The tint is green rather than white: that is not a bloom at all. It is free-floating algae, which behaves differently and clears differently. See green water.

Debris clears mechanically, usually within a day or two, and the filter does most of the work. A bloom needs its food supply cut off, and that means finding the trigger.

Step Two: What Happened in the Last Ten Days?

This is the section other pages skip in favor of a generic causes list, and it is the one that actually solves it. An established tank has a working biological filter. For the water to go milky, something must have delivered more organic material than that filter could process. Go backward.

Did something die? A fish you have not counted recently, or a snail, decomposing behind the filter or under a rock. This is the single most common cause of an unexplained bloom in a settled tank, and it is also the easiest to miss because a small body decomposes out of sight. Count your fish. Look under and behind hardscape.

Did you overfeed, or feed something new? A heavy feed, a holiday block, a well-meaning house guest, or a switch to a richer food all dump organics in at once. Overfeeding is the cause named most often across aquarium retailer and hobby guides, and it is also the easiest to fix. How much to feed aquarium fish covers what a reasonable amount actually looks like.

Did you disturb the substrate? Deep-vacuuming a bed that has not been touched in a long time, moving a large rock, replanting, or rescaping releases everything that has been quietly composting down there.

Did you clean or change the filter? This is the one that catches careful keepers. Rinsing all media at once, rinsing in tap water, or replacing a cartridge removes a large fraction of the bacterial colony. The tank then behaves like a partially cycled tank for a while, which can produce both a bloom and an ammonia reading. If your readings confirm it, an aquarium cycle crash explains what is happening and how to nurse it back.

Did you add fish, or has the stock grown? Fish grow, and a stocking level that was fine at purchase size may not be fine now. This is the slow version of the same problem, and the tell is that the cloudiness keeps coming back.

Did the water source change? After a water change, cloudiness can simply be dissolved gas coming out of solution, which looks like a fine mist of tiny bubbles and clears from the bottom up within an hour or two. That one is harmless and needs no action.

Step Three: Test, Because Cloudiness Is a Symptom

Whatever the trigger, the question that decides urgency is whether ammonia or nitrite is above zero.

• Both zero, fish behaving normally: you have an organic-load event, not a filtration failure. Find and remove the source, feed lightly for a few days, and let the filter catch up.
• Ammonia or nitrite above zero: the biological filter is not keeping up, whatever the reason. Now you do water changes, and you do them for the readings.
• Fish gasping at the surface as well as cloudy water: treat this as the priority. A dense bacterial bloom consumes oxygen, and in a warm, heavily stocked tank that can matter. Increase surface agitation and address the oxygen question first.

Algae and blooms do not make fish sick on their own. If fish have visible lesions, spots or damaged fins alongside the cloudiness, that is a separate problem happening at the same time, and when to contact an aquatic veterinarian covers where the boundary sits between what you can handle and what needs professional help.

What Not to Do

Do not strip the tank down. The instinct to give everything a thorough clean is exactly backward. Deep-cleaning the filter and the substrate at the same time removes the biological capacity you need and stirs up more of what caused the problem.

Do not stack products. A clarifier plus a bacterial supplement plus a treatment for something you have not diagnosed leaves you with water you cannot interpret. If you use a clarifier, use it knowing that it treats appearance, not cause.

Do not skip the count. Assuming all your fish are present is how a decomposing body goes unfound for a week.

The Honest Trade-off on Water Changes

In a new tank, water changes tend to prolong a bloom. In an established tank the calculation is different, because here you are usually removing the dissolved organics that are feeding it rather than interrupting a cycle that needs to finish.

I am not going to hand you a fixed percentage, because the right size depends on how far your readings are from zero and how large the disturbance was. Most keepers work somewhere in the range of a quarter to a half of the volume for a corrective change, and the reason for the upper limit is not the fish's tolerance of clean water but the speed of the change: swapping a large fraction at once moves temperature, pH and hardness faster than fish adjust comfortably. Temperature-matched, dechlorinated water, and gravel vacuuming the visibly dirty areas rather than the whole bed, is the safer version of the same intervention.

If the tank is new rather than established, the reasoning flips entirely, and cloudy water in a brand new tank is the page you want.

FAQ

How long does cloudy water last in an established tank? It depends entirely on whether the trigger is still present. Debris usually clears within a day or two once the filter catches it. A bloom persists as long as it is being fed, which is why finding the source matters more than any treatment.

My tank went cloudy after a water change. Did I do something wrong? Possibly not. A fine mist that clears from the bottom up within an hour or two is dissolved gas and is harmless. A milky haze that persists for days after a water change usually means filter media was cleaned at the same time, or that the substrate was disturbed.

Can cloudy water kill fish? The particles themselves do not. A heavy bacterial bloom can reduce dissolved oxygen, and the ammonia that often accompanies a filtration failure certainly can. Judge by your test results and by the fish's behavior, not by the water's appearance.

Should I add a bacterial supplement? It is a reasonable move if you have just lost filter media and have a real ammonia reading. It is not a fix for cloudiness in a tank that is testing zero on ammonia and nitrite.

Why does my tank keep going cloudy every few weeks? Recurrence points at a standing imbalance rather than an incident: too much food going in, more fish than the maintenance routine supports, or a filter that is undersized or cleaned too aggressively. Look at the pattern, not the individual episode.