How to acclimate fish: floating, drip acclimation and quarantine, step by step

To acclimate fish, float the sealed bag in the tank for 15 to 30 minutes so the temperature matches, then net the fish into the tank and throw the bag water away. Never pour bag water into your aquarium: it can carry ammonia, medicine and disease.

Fish that have spent many hours in a shipping box need a slightly different approach, because the water in the bag changes during the trip. Below you'll find the simple shop-to-home method, drip acclimation for sensitive animals, and why quarantine is the best acclimation of all.

What you need

A net, and a clean bucket or jug that has never had soap in it.
A clip or peg to hold the bag upright against the tank rim.
A thermometer.
For drip acclimation: a length of airline tubing and a valve or a loose knot to control the flow.
Ideally, a quarantine tank: a separate small tank with a heater and a cycled sponge filter.

When to take extra care

Normal: fish from a local shop, bagged for under an hour or two, going into water with similar pH and hardness. The basic float method is enough.
Take more care: fish shipped overnight by mail, fish from a shop with very different water to yours, shrimp, and delicate fish such as wild-caught tetras, discus or some plecos.
Emergency: fish lying on their side or gasping in the bag on arrival. Skip slow acclimation. Match the temperature quickly (about 10 minutes) and move them into clean, well-oxygenated water straight away.

Step by step: the float method

Turn the tank lights off, and dim the room. Darkness calms new fish and makes resident fish less likely to chase them.
Float the sealed bag on the tank surface for 15 to 30 minutes. This lets the water in the bag reach the tank's temperature. Clip it to the rim if it tips.
Check the temperature inside the bag and in the tank. They should be within about 1°C (2°F).
Net the fish out of the bag and into the tank. Pour the bag water into a sink or bucket, not the tank.
Leave the lights off for the rest of the day. Don't feed until the next day, and then only a little.
Watch for bullying over the next few days. Rearranging decor before adding fish helps break up old territories.

Many people add small cups of tank water to the bag every few minutes after floating. That's fine for fish that were only bagged for a short trip home. For long shipping, read the next section first.

Shipped fish: why the bag water is different

During a long trip, fish breathe out carbon dioxide (CO2) and release ammonia (their waste) into the bag. The CO2 makes the water more acidic, so the pH (a measure of how acidic water is) drops. That is actually helpful: at low pH most of the ammonia turns into ammonium, a much less toxic form.

The danger comes when you open the bag. The CO2 escapes, or you add your tank water, and the pH climbs. The ammonium turns back into toxic ammonia within minutes, while the fish are still sitting in it. University of Florida researchers note that high CO2 also makes it harder for fish blood to carry oxygen.

So for fish that have been bagged for many hours:

Float the bag unopened for 15 to 20 minutes to match the temperature.
Open the bag and move the fish straight into the tank or quarantine tank with a net or your wet hand.
Don't slowly mix tank water into the bag, and don't drip for hours.
Throw the bag water away.

If your water is very different from the seller's (for example, their pH is 6.5 and yours is 8.0), the best answer is a quarantine tank set close to the seller's water, then adjusted slowly over two to four weeks.

Step by step: drip acclimation

Drip acclimation slowly replaces bag water with tank water. It's widely used for shrimp, snails and sensitive fish that came from a nearby shop. It's less useful, and can be risky, for fish that have been in the bag for a long time (see above).

Float the bag to match temperature, then gently pour the fish and their water into a clean bucket or container. Tilt the container so the fish stay covered.
Run airline tubing from the tank into the container, start a siphon, and tie a loose knot or fit a valve so it drips about two to four drops per second.
When the water volume has roughly doubled, remove half and let it double again.
Many shrimp keepers drip for one to three hours. Fish usually need less, often 30 to 60 minutes.
Net the animals into the tank and throw the container water away.

Shrimp and sensitive fish

Shrimp are sensitive to quick changes in hardness and other minerals. Drip slowly, keep lights off, and don't panic if they hide for a day.
Snails can usually be added after floating. Check they're alive and closed up properly.
Wild-caught fish and many plecos come from soft, acidic water. Ask the seller what water they kept them in.
Bettas are often sold in tiny cups of old water. Float briefly and move them to clean water.

Quarantine first

A quarantine tank is the safest way to bring in new fish. The Merck Veterinary Manual recommends at least 30 days. During that time you can watch for ich, velvet, flukes and worms, treat them in a small tank, and let the fish recover from the trip. Quarantine plants and shrimp too, since they can carry snails, planaria and hydra.

Common mistakes

Pouring bag water into the tank.
Opening the bag of a long-shipped fish and slowly mixing in tank water.
Floating the bag under bright lights, which can overheat it.
Adding a lot of fish at once to a new tank. The filter's bacteria can't keep up (see cycling).
Skipping quarantine and adding sick fish straight to the display tank.
Feeding immediately. New fish rarely eat on the first day, and the food just rots.
Releasing a single new fish into a tank with established bullies (see aggressive fish).

Products that help

Quarantine tank with heater and sponge filter: the most useful upgrade for keeping new fish safe. Keep a seeded sponge in your main filter so it's always ready.
Drip acclimation kit or airline tubing with a valve: controls the drip for shrimp and sensitive fish.
Thermometer: confirms the bag and tank are matched.
Water test kit: checks ammonia, pH and hardness so you know how different your water is from the seller's.
Water conditioner that binds ammonia: useful in quarantine tanks for the first few days.
Battery air pump: adds oxygen if a long-shipped bag arrives with stressed fish.

Why it matters

Fish can't adjust quickly to sudden changes in temperature, pH or hardness. A shock on arrival weakens them, and a weak fish is more likely to get sick in its first weeks. That's why the first few weeks after a fish comes home are the riskiest, and why quarantine pays off.

When to get help

Contact the seller straight away if fish arrive dead or dying; most have a short window for claims and want photos of the unopened bag. If several new fish die in quarantine, or you see spots, fuzz or heavy breathing, read the guide to fish dying and ask an experienced shop, club or fish vet.

In the game

Multiple Tank Syndrome doesn't have an acclimation card yet, but the "Quarantine tank" card lets you hold new fish aside before they join a tank.

Sources

Crosby TC, Hill JE, Martinez CV, Watson CA, Pouder DB, Yanong RPE. On-farm transport of ornamental fish (FA119). UF/IFAS Extension.
Segrest Farms. Should I drip acclimate my fish?
Merck Veterinary Manual. Providing a home for fish.
Merck Veterinary Manual. Management of aquarium fish (quarantine of at least 30 days).