How to Add New Fish to a Tank (Acclimating Step-by-Step)

At a Glance

Add fish only to a cycled tank, a few at a time. Turn the lights off, float the sealed bag for about 30 minutes, net the fish across without pouring in the bag water, and keep the lights off for the rest of the day.

Here’s how to add new fish to a tank. Make sure the tank is cycled and turn the lights off. Float the sealed bag for about half an hour to match the temperature, then net the fish in and throw the bag water away. Add only a few fish at a time. This guide covers a freshwater tropical tank.

Scope: the steps below suit most community fish. Sensitive species, shrimp and marine fish often need a slower drip method, so check the care needs of what you buy. Moving fish you already own? See how to move fish to a new tank. More in the Fish Care guides. Keeping some ornamental species is restricted or banned in parts of Australia. The rules differ by state and change over time, so check your state Fisheries or Biosecurity department before you buy.

How to Add New Fish to a Tank: Before You Bring Fish Home

Cycle the tank first. A tank without established bacteria can’t handle the ammonia fish make. See how to cycle a fish tank, which takes about 4 to 6 weeks.
Plan a small first batch. RSPCA Australia says not to add large numbers of fish at any one time, because a gradual introduction lets the filter bacteria adapt.
Check the fit. Confirm the new fish suit your tank size, temperature and existing fish. The Stocking guides and best fish for beginners help.
Consider a quarantine tank. Many fishkeepers hold new fish apart for 2 to 4 weeks to watch for disease before they meet the main tank. It’s optional, but it protects the fish you already have.

How to Introduce New Fish to a Tank: The Float Method

This is the method most beginners need. It matches the temperature of the bag to the tank so the fish aren’t shocked by the change.

Test the tank. Ammonia and nitrite should read 0 and nitrate should be present.
Turn the lights off. Dim light calms the fish and cuts territorial squabbles.
Float the sealed bag. RSPCA UK says to leave the bag in the water for half an hour with the lights off, so the temperature evens out.
Throw the bag water away. RSPCA UK says to discard it, because it carries whatever the shop’s tank had, waste and possibly disease.
Leave the lights off for the rest of the day. RSPCA UK says to keep the tank lights off for the rest of the day, so the fish can settle.

Australia-specific

In an Australian summer, a bag left in a hot car can overheat well before you get home. Ask the shop to bag the fish in an insulated bag, keep it out of the sun, and go straight home. Shop water can also differ from your tap in hardness and pH, so ask the shop what theirs is.

How to Add New Fish to a Tank Slowly: The Drip Method

Some keepers prefer a drip method for sensitive fish, invertebrates, or when your water differs a lot from the shop’s. The steps are general hobby practice rather than an RSPCA procedure.

Pour the fish and their bag water into a clean bucket used only for the tank.
Run a length of airline tubing from the tank to the bucket with a knot or a valve to control it.
Let tank water drip in at 2 to 3 drops a second until the bucket’s volume has roughly doubled, usually 30 to 60 minutes.
Net the fish into the tank and discard the bucket water.

After You Add the Fish

Test daily for the first week. A new batch adds ammonia, so check ammonia and nitrite. If you see a reading, see how to reduce ammonia in a fish tank.
Feed lightly. Many keepers wait until the next day, and then give a small portion.
Watch the newcomers. Chasing, hiding for a day or two, and less appetite are common. Constant harassment or fin damage is not.
Wait before the next batch. Let the tank settle for a couple of weeks and confirm ammonia and nitrite stay at 0.

Common Mistakes

Adding fish to an uncycled tank. Ammonia will build up with nothing to remove it.
Pouring the bag water in. It brings waste and potential disease.
Buying a full stock list at once. The filter bacteria can’t keep up.
Leaving the lights on. Bright lights add stress during the move.
Skipping the temperature match. A sudden difference stresses fish.

When to Call It

If a new fish is gasping at the surface, clamped, covered in spots or still hiding after several days, test the water first. Then ask your aquarium shop, or a vet who treats fish. If ammonia or nitrite is above 0, treat that as the priority.

Related Guides

Keep reading on the same topic:

Frequently Asked Questions

How long should I acclimate new fish?

Float the sealed bag for about 30 minutes, according to RSPCA UK. Sensitive fish or a big water difference may call for a slower drip of 30 to 60 minutes.

How many fish can I add at once?

Only a few. RSPCA Australia says not to add large numbers at any one time, so the filter bacteria can adapt. Retest ammonia and nitrite before the next batch.

Should I put the shop water in my tank?

No. RSPCA UK says to discard the bag water, because it can carry waste and disease from the shop’s system.

Do I need to turn the lights off when adding fish?

Yes. Turn them off before you float the bag, and leave them off for the rest of the day, as RSPCA UK advises.

Can I add new fish to a tank that already has fish?

Yes, if the tank is stable and the species get along. Many keepers quarantine newcomers for 2 to 4 weeks first, because disease can spread to the fish you already have.