Why acclimation matters
The water your new fish travelled in almost never matches your tank. Temperature, pH and hardness can all differ, and fish are cold-blooded animals that cannot tolerate sudden change. Dropping a fish straight from the bag into different water causes shock — a stress response that can be fatal, or leave the fish weakened and prone to disease days later. Acclimation is simply the process of letting the fish adjust to your water gradually instead of all at once.
The float method (temperature)
The simplest method matches temperature and is fine for hardy fish moving between similar water:
- Float the sealed bag in your tank for about 15–20 minutes so the bag water gradually reaches tank temperature.
- Open the bag and, if you wish, add a little tank water to begin blending parameters.
- Net the fish out and release them into the tank — do not pour in the bag water.
Floating handles temperature but does little for differences in pH or hardness, so for anything sensitive, use the drip method below.
The drip method (parameters)
Drip acclimation is the gentler, more thorough approach — ideal for sensitive fish, wild-caught species and all invertebrates like shrimp. It slowly introduces your tank water so the fish adjust to parameters over time:
- Empty the bag (fish and its water) into a clean container placed below the tank.
- Set up a siphon from the tank using airline tubing, and tie a loose knot in it to slow the flow to a gentle drip — a few drips per second.
- Drip for 30–60 minutes until the container volume has roughly doubled or tripled, so the water is now mostly from your tank.
- Net the fish out and place them in the aquarium, discarding the container water.
Quarantine — the gold standard
The safest practice, especially once you have an established community, is to acclimate and hold new fish in a separate quarantine tank for two to four weeks before adding them to your main aquarium. This lets you spot illness before it can spread to healthy fish. A simple, sponge-filtered quarantine tank is inexpensive and can save an entire stock from an introduced disease. It is optional for a first fish going into an empty cycled tank, but invaluable thereafter.
After they are in
Leave the lights off for a few hours and do not feed on the first day — new fish rarely eat straight away and uneaten food only fouls the water. Watch them quietly over the next few days for normal swimming, colour and appetite. Keep up your usual water changes and maintenance, and add new arrivals gradually so your filter keeps pace — see our guide on how many fish you can keep for stocking sensibly. Good food helps them recover from the move, too; our best fish food guide covers the essentials.
If a fish looks unwell in the days after introduction, keep your response general and calm — maintain excellent water quality, reduce stress, and seek advice from an aquatics specialist rather than guessing at treatments. Careful acclimation is your best insurance that new fish settle in healthy and stay that way.