Here is how to acclimate new fish in one paragraph. Float the sealed bag in your tank so the water inside slowly reaches your tank's temperature. Then open it and mix small amounts of your tank water into the bag water over a period of time, so the fish adjusts to your pH and hardness gradually rather than all at once. Finally, net the fish out and put it in the tank. The bag water itself does not go in. It did its job on the trip home, and pouring it in adds the store's waste and whatever lives in it to your tank.
My name is Hossain. On this site I explain why before I tell you what, and acclimation is a good example of a step people copy without knowing what it is for. Once you know the reason behind each part, you can tell which part matters for the fish in your hand.
What Acclimation Is Actually Doing
The reader question behind this article is usually "How long do I float the bag, and do I pour it in?" Both halves of that question make more sense once you know what acclimation is.
University of Florida IFAS Extension, in its publication FA119, On-Farm Transport of Ornamental Fish, defines acclimation as "the process of slowly introducing the fish to different quality water to allow physiological adjustments to occur gradually over time." I read that publication on September 28, 2026.
The key word is gradually. A fish's body is tuned to the water it is sitting in. Its gills, its skin and its internal salt balance all work against that particular temperature, pH and mineral content. Your tank water is almost never identical to the store's water. Acclimation is not a ritual. It is a slow handover from one set of water conditions to another, so the fish is not asked to adjust to everything in one second.
Two things change between the bag and your tank, and they are handled in two different steps:
- Temperature, which floating the bag takes care of.
- Water chemistry (pH, hardness and dissolved minerals), which mixing water takes care of.
Step 1: Floating the Bag, and What It Does Not Do
Floating the sealed bag in your tank lets the water inside slowly warm or cool to the temperature of your tank without the fish touching your water yet. That is all it does. It does not adjust the fish to your pH or hardness, because no water is being exchanged.
There is no magic number of minutes. The bag is ready when the water inside no longer feels noticeably warmer or cooler than the tank. A small bag carried across a warm room gets there quickly. A bag that sat in a cold car on the way home takes longer.
UF/IFAS gives a sense of when temperature really matters. FA119 says fish "must be slowly acclimated if there are significant differences in transport water and receiving water temperature," which it defines as more than 5.5 °C (10 °F). It adds that if the difference is greater than 10 °C, acclimation should ideally proceed over two or more hours. Those figures come from fish-farm transport, not home tanks, but the principle carries over: the bigger the gap, the slower you go.
A few practical points while the bag floats:
- Turn the tank light off. A dim tank is calmer for a fish that has just been netted, bagged and carried.
- Keep the bag away from direct light and away from the heater. You want it to follow the tank's temperature, not a hot spot.
- Do not open the bag yet. The reason is chemical, and it is the part most people skip.
Why You Should Not Open the Bag Too Early
This is the part of acclimation that explains most of the confusing advice online, so it is worth a minute.
Inside a sealed bag, the fish keeps breathing and producing waste. Two things build up: carbon dioxide from breathing, and ammonia from waste. UF/IFAS FA119 explains that unionized ammonia (NH3) is much more toxic than ionized ammonia (NH4+), that ammonia shifts toward the more toxic form as pH and temperature rise, and that carbonic acid formed from carbon dioxide lowers pH and shifts ammonia toward the less toxic form.
Put those together and you get a surprising result. In a sealed bag, the building carbon dioxide pulls the pH down, and that lower pH keeps most of the ammonia in its less harmful form. The bag is, in a sense, protecting the fish from its own waste.
When you open the bag, carbon dioxide escapes to the air. The pH starts rising. As it rises, more of that stored ammonia shifts into its toxic form. For a fish that was bagged for a short trip home from a local shop, there has not been time for much ammonia to build up, and this barely matters. For a fish that spent a day or more in a shipping box, it can matter a great deal.
If pH itself is new to you, why stable pH matters more than the exact number is a good companion to this section.
How to Acclimate New Fish by Mixing Water (the Standard Method)
Once the temperature has evened out, the next job is chemistry. The usual method looks like this:
- Open the bag and roll the top down so it floats, or carefully move the fish and its bag water into a clean container that has never had soap or chemicals in it.
- Add a small amount of your tank water. Enough that the fish is still covered and the container is not overflowing.
- Wait, then add a little more. Repeat several times. Each addition moves the water in the container a bit closer to your tank's water.
- Net the fish out once the container is mostly tank water, and release it into the tank.
- Throw the container water away. Down the sink, not into the aquarium.
How long to spend depends on how different the two waters are. If your tap water and the store's water are similar, a short acclimation is fine. If your water is much harder or softer than the store's, or your pH is quite different, spread the additions out over a longer time. What matters is that the change happens in steps, not in one pour.
Before you buy, it helps to ask the store what pH and hardness their tanks run at. If they know, you have a real idea of how big the gap is. If they do not, go slower.
Drip Acclimation: When It Is Worth It
Drip acclimation is the same idea as the standard method, done more smoothly. You put the fish and its bag water in a container below the tank, run a length of airline tubing from the tank into the container, and tie a loose knot in it (or use a small valve) so tank water drips in slowly. Over time the container fills with tank water, a drop at a time, instead of in a few big splashes.
It is not magic, and most hardy community fish do not need it. It earns its place when:
- Your water is very different from the store's water, especially in hardness or pH.
- The animal is known to be sensitive to change. Dwarf shrimp are the classic example; they tend to be far less forgiving of sudden changes than most community fish.
- You want the smoothest possible change and have the time to watch it.
Two cautions. Watch the container so it does not overflow onto the floor. And check the drip after a few minutes, because airline knots loosen and tighten on their own.
Fish That Were Shipped: The Special Case
Fish ordered online can spend a day or more in a sealed bag. That is exactly the situation where the ammonia and pH chemistry above becomes serious, because there has been plenty of time for waste to build up.
The concern with a long, slow acclimation for these fish is simple. Opening the bag lets carbon dioxide escape and the pH rise, but mixing in tank water only dilutes the ammonia gradually. So for a while, the fish sits in water where the ammonia is still concentrated and its toxic share is growing.
This is why some sellers of shipped fish recommend a faster approach, sometimes called "plop and drop": match the temperature, then move the fish out of the shipping water and into the tank (or a quarantine tank) fairly quickly, leaving the shipping water behind. I am not going to tell you that one method is right for every shipment, because the seller knows how long their fish were in transit and how they were packed. If your seller includes acclimation instructions, follow them. They are written for that exact package.
Wherever the fish came from, a separate tank for the first few weeks is the step that protects the fish you already own. Why quarantine is the cheapest insurance in the hobby explains what it catches that acclimation cannot.
What the Bag Water Is For, and Why It Stays Out of Your Tank
The bag water had one job: keep the fish alive, in water it was already used to, on the way to your house. Once the fish is out, that job is done.
There are three reasons not to pour it in:
- It carries the trip's waste. Ammonia built up in that small volume of water, and you do not want to add it to your tank.
- It can carry disease. Parasites, bacteria and fungus that are in the store's system travel in the store's water. A fish can look healthy while the water it came in does not.
- It carries whatever was added at the store. Some stores treat their systems. You cannot know what is in that water or how much, and you do not want it in your tank by accident.
The only sensible destination for bag water is the sink.
After the Fish Goes In
The first hours matter too.
- Keep the light off for the rest of the day. A calm, dim tank helps a new fish settle and find cover.
- Do not feed straight away. Many fish will not eat until they have settled, and uneaten food only adds waste. A small feeding the next day is plenty.
- Expect hiding. A new fish that tucks itself behind a plant or rock for a day is normal behavior, not a sign something is wrong.
- Watch for the signs that are not normal. Gasping at the surface, lying on the bottom breathing fast, or losing balance are different from shyness.
If a new fish looks seriously unwell after acclimation, do not reach for a medicine on a guess. Test your water first so you know what you are working with, and if the fish is in distress, talk to an aquatic veterinarian. A vet can tell you what is actually happening, which no blog post, including this one, can do from a description.
Common Acclimation Mistakes
- Floating the bag and then pouring everything in. This handles temperature and nothing else, and it adds the bag water.
- Opening a long-shipped bag and then acclimating for a very long time. This is the one situation where "slower is safer" can backfire, for the chemistry reasons above.
- Leaving the bag in bright light or next to the heater. It overheats the water the fish is sitting in.
- Adding several new fish to a tank that is not ready. Acclimation cannot fix an uncycled tank. If this is your first stock, read how to add your first fish without restarting the cycle before you buy.
FAQ
How long should I float the bag before releasing a fish?
Long enough that the water in the bag feels the same temperature as your tank. There is no fixed number of minutes. If the gap is large, UF/IFAS Extension FA119 advises going much slower, and floating alone only handles temperature, not water chemistry.
Should I pour the bag water into my tank?
No. The bag water carries ammonia from the trip, anything that lives in the store's system, and anything the store added to its water. Net the fish out and throw the bag water away.
Is drip acclimation better than the standard method?
It is smoother, not automatically better. It is worth the effort when your water is very different from the store's or when the animal is sensitive, such as dwarf shrimp. Most hardy community fish do well with the standard method.
Why do some sellers say to acclimate shipped fish quickly?
Because a long-shipped bag holds built-up ammonia, and the low pH in the sealed bag keeps most of it in a less toxic form. Opening the bag raises the pH and makes that ammonia more harmful. If your seller gives instructions, follow them.
My new fish is hiding. Did acclimation go wrong?
Probably not. Hiding for a day or so is normal after a move. Gasping, rapid breathing on the bottom or loss of balance are different, and are a reason to test your water and contact an aquatic veterinarian.