How to acclimate fish: float, cup and drip methods step by step

To acclimate fish from a local store, float the sealed bag for about 15 minutes to match temperature, then mix in tank water over 30–45 minutes and net the fish into the tank without the bag water. Fish shipped overnight should skip the slow mix: match temperature, then net them straight in, because opening the bag makes its built-up ammonia more toxic. Shrimp and other sensitive invertebrates need a slow drip of 1–2 drops per second for 1–2 hours.

Acclimation gets a fish from the water in its bag to the water in your tank without shocking it. The two things that hurt fish on the way in are sudden temperature changes and sudden changes in water chemistry (pH, hardness and ammonia). Which method you use depends on how long the fish spent in the bag and what kind of animal it is.

Acclimation only helps if the tank is ready. New fish should go into a fully cycled tank that reads 0 ammonia and 0 nitrite. If you aren’t sure, read how to cycle a fish tank first.

Which method to use

Animal and situation	Method	Time
Fish from a local store (in the bag under 2–3 hours)	Float, then cup method	45–60 minutes total
Fish shipped overnight or longer	Float, then net straight in (“plop and drop”)	10–15 minutes (30–60 on the lid for breather bags)
Shrimp (cherry, Amano, crystal and bee) and snails	Float, then drip	1–2 hours, longer for crystal and bee shrimp
Fish moving to very different water (for example hard to very soft)	Float, then drip	1–2 hours, with an ammonia binder if shipped
Fish moving from your quarantine tank to your main tank	Match temperature, net across	15–30 minutes

Every method fixes temperature first, while the bag is still sealed, then changes water chemistry as slowly as the animal needs, but a slow change only helps when the bag water is clean.

Before you bring fish home

Test the tank. Ammonia and nitrite should both read 0. Write down the pH and temperature so you can compare.
Have the kit ready. You need a clean bucket or plastic container used only for fish, a net, water conditioner and a thermometer. For shrimp, add airline tubing and a small valve.
Turn the tank lights off. Leave them off from now until the next morning, or for at least 6–8 hours.

The float-and-cup method for store-bought fish

Use this for fish bagged at a local store a few hours ago at most, when the bag water has had little time to build up ammonia.

Float the sealed bag in the tank for 15 minutes. This brings the bag water to tank temperature. Don’t open it yet, and don’t leave it floating under a bright light for more than about 30 minutes.
Open the bag and pour the fish and their water into a clean container. Use just enough bag water to cover them.
Add tank water equal to the water in the container, so it doubles in volume. A cup or a turkey baster works well. Wait 15 minutes.
Double the volume again with tank water and wait 10 minutes. Pour some water away if the container gets too full.
Double it a third time and wait 10 minutes. The water is now about one-eighth bag water.
Net the fish into the tank. Lower the net into the water and let them swim out.
Throw away the container water. Never pour it into the tank, and rinse the container and net afterwards.

Fish shipped overnight: why the bag water burns gills

Fish keep producing ammonia during shipping, and in a sealed bag it has nowhere to go. Their breathing also builds up carbon dioxide, which pushes the pH of the bag water down. At low pH most of that ammonia sits in its ionized form (ammonium), which is far less toxic.

Opening the bag lets the carbon dioxide escape, so the pH rises and more of the ammonia turns into its toxic form. LSU AgCenter extension advice says this happens quickly, and faster still if the water is splashed or aerated. Mixing in higher-pH tank water adds to the jump, which is why a long drip in shipping water can kill fish that survived the trip. Short trips are less risky, because less ammonia has built up.

A Southern Regional Aquaculture Center fact sheet (SRAC 463) shows how sharp the change is: each 1-unit rise in pH makes about 10 times more of the ammonia toxic. It was written for pond fish, but the chemistry is the same in a tank. Here is the toxic share at 79 °F (26 °C), from its table:

pH	Share of total ammonia that is toxic	Toxic ammonia if the test reads 2 ppm
7.0	0.6%	0.012 ppm
7.4	1.5%	0.03 ppm
7.8	3.7%	0.07 ppm
8.0	5.7%	0.11 ppm
8.2	8.8%	0.18 ppm

The same fact sheet says toxic ammonia from about 0.6 ppm can kill fish within a few days, and long-term exposure to as little as 0.06 ppm can damage the gills. So a 2 ppm reading at pH 7.0 is about a fifth of that lower level, while at pH 8.0 it is nearly twice it. In your own tank, aim for 0 ppm at any pH. The ammonia guide covers testing and emergencies.

Plop and drop, step by step

Dim the room and keep the tank lights off. Shipped fish are exhausted.
Match temperature with the bag still closed. Float sealed bags for 10–15 minutes. Aquarium Co-op suggests resting breather bags (thin plastic that lets gas pass through) on the tank lid for 30–60 minutes instead of floating them. Follow any instructions the seller sends.
Set a net over a bucket and cut the bag open over it. The water drains away and the fish stay in the net. Don’t splash or aerate the bag water first.
Lower the net into the tank right away and let the fish swim out.
Discard the shipping water, plus any fish that died in transit, without letting it touch the tank.

If the shipping water is far from your tank’s pH or hardness and you want to drip anyway, add an ammonia-binding water conditioner to the container as soon as the bag is open, dosed as its label directs for that volume.

Drip acclimation for shrimp and sensitive animals

Shrimp feel changes in pH, hardness (GH and KH) and total dissolved solids (TDS) more than fish do. A fast change may not kill them on the day, but it can cause a failed molt days later, which is one reason new shrimp sometimes die a week after purchase. Cherry shrimp are among the hardiest, and they still do better with a drip of about an hour or more.

Float the sealed bag for 15 minutes to match temperature, or rest a breather bag on the lid for 30–60 minutes.
Pour the shrimp and their water into a container that can hold at least four times that amount. If they were shipped, add an ammonia-binding conditioner now, following its label for the volume in the container.
Set up the drip. Put one end of a length of airline tubing in the tank and secure it. Start a siphon, then control the flow with a small valve, or tie a loose knot in the tubing and tighten it.
Set the rate to 1–2 drops per second. Keep the container below the tank’s water line so the siphon keeps running.
Drip for 1–2 hours until the container holds about two-thirds to three-quarters tank water, roughly three to four times the starting volume. Go slower and longer for crystal and bee shrimp, which need soft water, or if your water is very different from the seller’s.
Net or spoon the shrimp into the tank. Don’t pour in the container water.

If your room is cold, stand the container in a tub of warm water so it doesn’t cool during the drip. A TDS meter helps: when the bag and tank readings are far apart, drip longer.

The first 24 hours

Don’t feed the new fish on day one. Feed the residents a small meal just before the newcomers go in, so they are busy eating instead of chasing.
Expect some hiding. Pale color and staying out of sight are normal for a day or two.
Test ammonia and nitrite the next day, and again 2–3 days later. Every new fish adds waste, and the filter bacteria need a few days to catch up. If either reads above 0, change 25–50% of the water with dechlorinated, temperature-matched water, or 50% the same day if it reads 0.5 ppm or more.

Warning signs that are not normal: gasping at the surface, rapid gill movement, lying on the bottom, clamped fins that last past the first day or two, or red or inflamed gills. Test the water at once and change water if ammonia or nitrite shows up. White spots like grains of salt in the first two weeks are usually ich: see the ich guide. If a fish keeps getting worse in clean water, or several die, contact an aquatic vet.

Quarantine tank basics

A quarantine tank is a small, separate tank where new fish live for a few weeks before they meet your main tank. Ich, flukes and bacterial infections such as columnaris often arrive with new fish, and they are much easier to treat in 10 gallons than in your whole tank.

A simple setup, based on the Merck Veterinary Manual and Aquarium Co-op:

A 10-gallon tank (38 L), or a clear storage tub with a lid.
A sponge filter that has been running in your main tank for a few weeks, so it is already cycled.
A heater, thermometer and air pump.
A bare bottom, so you can see droppings and leftover food, and a pot or pipe to hide in.
Its own net, siphon and bucket. Work on the quarantine tank last and wash your hands afterwards.

How long: The Merck Veterinary Manual calls 30 days the minimum. Aquarium Co-op suggests 4–6 weeks counted from the last sign of illness or death. In other words, the clock restarts if a fish gets sick.

What to check every day: whether each fish eats, swims and breathes normally, and any spots, fuzz, sores, cloudy eyes or stringy white droppings. Do a 25% water change weekly. Treat only when you know what you’re treating, follow the medication label exactly, and take carbon out of the filter while you medicate. If a fish dies or you can’t tell what is wrong, an aquatic vet can find the cause before it spreads.

Quarantine has limits. Merck notes it is poor at detecting mycobacteria, many viruses and microsporidia (the group behind neon tetra disease), so buying healthy fish still matters. Afterwards, match the temperature and net the fish across, since both tanks use the same tap water.

Stocking order: who goes in first

Fish added early claim the tank, so add peaceful fish first and territorial fish last.

Stage	What to add	Why
1	Hardy, peaceful schooling fish (for example harlequin rasboras or zebra danios)	They cope with the small swings of a newly cycled tank
2	Peaceful bottom dwellers (for example bronze corydoras, kept in a group of 6 or more)	They use a different part of the tank and rarely compete with midwater fish
3	Sensitive grazers, shrimp and snails (otocinclus, cherry and Amano shrimp)	They need a mature tank with algae and biofilm. Wait at least 3 months for otocinclus
4	Territorial or semi-aggressive fish (dwarf gourami, rams, kribensis, angelfish, betta)	Arriving last means they can’t claim the whole tank before the others settle

A few practical rules:

Add in batches. Add no more than about a quarter of your planned stock at once. Test ammonia and nitrite every day or two for the first week, and wait about 2 weeks before the next batch, adding more only when both stay at 0.
Add a school in one go if the tank allows. A group of 8 harlequin rasboras added together settles faster than a few at a time. In a small tank, make them a batch of their own.
Rearrange the decor before adding a territorial fish to an established tank. It breaks up old territories.
Check the plan before you buy. The stocking calculator shows whether your planned fish fit the tank, and the compatibility checker flags pairs that clash.

Don’t chase exact water numbers

Most farm-bred community fish adapt to a wide range of pH and hardness, so acclimation is about avoiding sudden change, not matching the store’s water exactly. Stable water matters more than a perfect pH. See water parameters before reaching for pH chemicals.

Gear mentioned in this guide

Drip acclimation kit (airline tubing and valve) (opens Amazon in a new tab)Lets new fish and shrimp adjust to your water slowly. How to choose
Small quarantine tank (10 gallon) (opens Amazon in a new tab)Keep new fish apart for about 4 weeks to protect your main tank. How to choose
Fish net (opens Amazon in a new tab)A fine, soft net is gentler on fins.

Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.

Frequently asked questions

How long should I float the bag before releasing fish?

About 15 minutes is enough to match temperature for a typical store bag. Don't leave a sealed bag floating under the light for more than about 30 minutes, because it can overheat and run low on oxygen.

Should I pour the bag water into my tank?

No. Bag water can carry ammonia, medications and disease from the store or shipper. Net the fish out and throw the water away.

Do I need to drip acclimate fish?

Most store-bought fish do fine with the float-and-cup method. Use a drip for shrimp, snails, and fish going into water very different from what they came from. For fish shipped overnight, avoid a long drip in opened bag water unless you first add an ammonia-binding conditioner, dosed as its label directs.

How long should the aquarium lights stay off after adding fish?

Keep them off for at least 6–8 hours, and simply leaving them off until the next morning is easiest. Dim light lowers stress and gives new fish time to find hiding places before the residents notice them.

When can I feed new fish?

Wait until the next day, then give a small meal. Many new fish won't eat on day one, and uneaten food only adds ammonia to a tank that just took on extra waste.