How to Introduce New Fish to Your Tank Safely
Quarantine first
A quarantine tank does not need to be pretty: a 10-gallon tank, a sponge filter seeded in your main filter, a heater and somewhere to hide. Run new arrivals in it for two weeks minimum, four if you can. Watch for white spots, clamped fins, stringy white waste, rapid gilling and any refusal to eat.
The reason this matters: shop tanks are usually on a shared water system, so one sick fish anywhere in the store exposes everything. A fish can look perfectly healthy and still be incubating ich or a bacterial infection for a week.
Acclimation, step by step
- Dim the lights in the receiving tank and the room.
- Float the sealed bag for 15–20 minutes to equalise temperature.
- Drip acclimate. Open the bag into a clean container, run airline tubing with a loose knot from the tank, and drip until the volume has roughly tripled — about 45–60 minutes. This is mandatory for shrimp and snails, and strongly advised for all fish.
- Net the fish into the tank. Never pour shop water in — it carries whatever the shop system carries.
- Leave the lights off for the rest of the day and do not feed until the next morning.
Preventing aggression on introduction
- Rearrange the hardscape before adding a fish to a tank with established territories. It resets the map and everyone renegotiates on equal footing.
- Add at feeding time — residents are distracted.
- Add whole groups at once, not one shoaling fish at a time.
- Add the most territorial species last in a new tank.
The first two weeks after introduction
Test ammonia and nitrite every other day. Adding stock is the classic trigger of a mini-cycle, because the bacteria colony is sized to the previous bioload and takes one to two weeks to expand. Feed lightly and check that every individual is eating — a fish that will not eat after four days needs attention.
Frequently asked questions
How long should I quarantine new fish?
Two weeks minimum, four weeks ideally. Most parasites and bacterial infections show themselves within that window, and treating one small quarantine tank is far easier than treating a stocked display tank.
How do I acclimate new fish?
Float the sealed bag 15–20 minutes to match temperature, then drip acclimate over 45–60 minutes until the water volume roughly triples, then net the fish in. Never pour shop water into your tank.
Can I add all my fish at once?
In a fully cycled tank that was cycled to a full ammonia load, yes within reason. In any other case add one group at a time with two weeks between additions.
Why is my new fish hiding?
It is normal for the first two to five days. Keep the lights dim, do not chase it with a net, and make sure there is dense cover. If it is still hiding after a week, check water parameters and whether a tank mate is harassing it.
Next steps
Written and reviewed by Marcus Rivera. See our methodology for how figures on this site are derived.