Something is wrong in your tank and you need to act now. This page is the short version: what to check first, what to do, what to avoid, and when to stop reading websites and contact a professional.
This page is educational and is not veterinary advice. It cannot examine your fish. If your animals are seriously ill or injured, contact a qualified aquatic veterinarian — the section "When To Contact a Professional" below covers when that is the right call. Our disclaimer sets out the limits of what we can offer.
Start Here: Test the Water
Most sudden aquarium emergencies are water quality problems, not diseases. Fish that are gasping, listless, clamped, flashing, or dying without visible marks are very often telling you about the water rather than about an infection.
Before you treat anything, test for ammonia, nitrite, nitrate, temperature and pH — in that order of urgency. Ammonia and nitrite should both read zero. Anything above zero is an active emergency for the animals.
Do this before you add a single medication. Treating a water quality problem with medicine makes things worse: many treatments stress fish further and reduce dissolved oxygen, and none of them fix the underlying cause.
If you have no test kit, a partial water change is still the safest first move while you get one.
The Universal First Response
For most emergencies, before you have identified the exact cause:
- Stop feeding. Uneaten food decays into more ammonia. Healthy fish are fine for several days without food. This costs you nothing and removes one variable.
- Do a partial water change — around 25–50% — with water that is dechlorinated and matched to the tank's current temperature.
- Increase aeration. Add an air stone, or raise the outlet flow to break the surface. Distressed fish and warm water both mean less available oxygen.
- Do not clean the filter. Whatever else is wrong, the bacteria in your filter are what is keeping the tank alive.
- Remove anything obviously decaying — a dead fish, a rotting plant, uneaten food.
- Write down what you observe and what you did, with times. If you end up speaking to a vet, this record is the most useful thing you can bring.
Then work out the cause before doing anything more aggressive.
What Not To Do in an Emergency
Panic causes more losses than the original problem. In particular:
- Do not change 100% of the water. A dramatic swing in chemistry and temperature can kill fish that the original problem would not have.
- Do not rinse or replace filter media in untreated tap water. Chlorine kills your biological filter, and a dead filter turns a bad day into new tank syndrome.
- Do not replace all your filter media at once, even with clean water. Replace one part at a time, weeks apart.
- Do not add untreated tap water. Chlorine and chloramine are lethal to fish and to your filter bacteria. Always use a dechlorinator.
- Do not "shotgun" medications. Adding several treatments hoping one works is a common way to finish off already-stressed fish. Many medications interact badly, and copper-based treatments are lethal to shrimp and snails.
- Do not change temperature quickly. Correct a bad temperature gradually, over hours.
- Do not use household soap, detergent or cleaning products on anything that touches the tank, ever.
- Do not restock or add new fish while the tank is unstable.
Common Emergencies
Ammonia or nitrite above zero
The most common true emergency, and the most common killer of new tanks. Both are toxic at any detectable level.
Do a partial water change immediately and repeat as needed to bring levels down. Stop feeding. A water conditioner that detoxifies ammonia buys time but does not fix the cause. Do not touch the filter. Then find the source: an uncycled tank, overfeeding, overstocking, a dead fish hidden behind the hardscape, a filter that was cleaned too thoroughly, or a heater or filter failure that killed off the bacteria.
Fish gasping at the surface or breathing rapidly
Treat as urgent. Usually low dissolved oxygen, ammonia or nitrite poisoning, or occasionally a gill problem.
Increase surface agitation and aeration immediately, test the water, and do a partial water change. Check the temperature — warm water holds less oxygen, so a heater stuck on can present exactly this way. Note whether it is the whole tank or one fish: the whole tank points to water or oxygen, one fish points to that individual.
Heater failure
A heater stuck on cooks a tank; a heater that has failed off lets it chill. Both are dangerous, and a cracked heater in the water is also an electrical hazard.
Unplug it. Correct the temperature slowly — floating sealed bottles of cool or warm water is gentler than pouring water in. Never pour hot water directly into a stocked tank.
Power outage
The filter bacteria and the oxygen supply are your concerns, not the temperature, in the first hours.
Do not feed. A battery air pump is the single most useful thing you can own for this; failing that, gently pour water with a cup periodically to move the surface. Insulate the tank with blankets to hold heat. If power is out for many hours, the filter media may lose oxygen and begin to die — when power returns, watch for an ammonia spike and consider rinsing the media in tank water before restarting a filter that has been off for a long time.
Wrong or overdosed treatment
Do a large partial water change and run activated carbon to strip the medication out, then reassess. Do not add a second medication to counteract the first.
A fish is visibly injured, ulcerated or growing something
This is where the internet stops being useful. Wounds, ulcers, lesions, growths and anything eroding into the body are exactly what a veterinarian should see. Keep the water clean and stable, reduce stress, and get professional input rather than working through medications one at a time.
When To Contact a Professional
Please contact a qualified aquatic veterinarian or an experienced aquatic animal health professional when:
- Several fish die in quick succession, especially with good water parameters
- A fish has open wounds, ulcers, lesions, growths or bleeding
- Symptoms continue after water quality has been corrected and stable for several days
- Fish are behaving in ways you cannot explain — spinning, convulsing, unable to stay upright
- You are considering a treatment you do not fully understand
- The animal appears to be suffering and you are weighing euthanasia
- It is a valuable, large or long-lived animal, such as a koi or a mature marine fish
Aquatic medicine is a genuine veterinary specialism. Not every general practice vet treats fish, but many countries have vets who do, and some will consult by phone or video with photographs.
Your local aquarium society or a long-established independent fish shop is often the fastest route to finding someone locally who treats fish. It is worth finding out who that is before you need them.
What To Have Ready Before You Call
You will get far better help if you can answer these:
- Water parameters with numbers: ammonia, nitrite, nitrate, pH, temperature — and when you tested
- Tank size, how long it has been running, and whether it is fully cycled
- What lives in it — every species and how many
- What changed recently — new fish, new food, a water change, medication, a cleaned filter, a new decoration, building work or aerosols near the tank
- Symptoms and when they started, and how many animals are affected
- Clear photographs or a short video of the affected fish, if you can manage it
The Emergency Kit
Most aquarium emergencies are survivable if you have these already, and much harder if you have to go shopping mid-crisis:
- A liquid test kit for ammonia, nitrite, nitrate and pH
- Dechlorinator, ideally one that also detoxifies ammonia
- A battery-operated air pump and spare batteries
- A spare heater
- A thermometer
- Buckets and a siphon used only for the aquarium — never for anything else
- Activated carbon, to strip medication back out
- A quarantine or hospital tank, even a simple one
The Real Lesson
Almost every emergency on this page traces back to the same few causes: a tank that was never properly cycled, a tank with more fish in it than it can process, a filter that was cleaned too well, or a change made too fast.
Stable water, appropriate stocking, patient cycling and routine maintenance are not the boring part of the hobby. They are the part that means you never have to read a page like this again.
When the immediate crisis has passed, those four causes are what to read up on next. Guides on aquarium care, water quality and fish health are being added to this site over time; our editorial policy explains the standards they are held to, and you can reach us any time through our contact page.