The short answer
Introduce new fish slowly and cleanly. Ideally quarantine them in a separate tank first, then acclimate each fish to your water temperature and chemistry using the floating or drip method before netting them in. The two things to avoid are shock (sudden temperature or parameter change) and disease (adding unquarantined fish straight to your main tank).
Acclimation, step by step
Give the fish time to adjust rather than dropping them in:
- Float the bag in the tank for 15–20 minutes so temperatures equalise.
- Add tank water gradually — mix a little of your tank water into the bag every few minutes over 20–30 minutes, so the fish eases into your parameters (this is drip/float acclimation).
- Net the fish into the tank and discard the bag water rather than pouring it in — it can carry disease and poor-quality shop water.
- Dim the lights for the first hours to reduce stress while the fish settles.
For a fuller walkthrough, see how to acclimate new fish.
Quarantine first
The safest keepers run a small quarantine tank. Keep new arrivals there for around two weeks so any illness shows up where it can’t spread to your main fish. It also lets nervous newcomers eat and settle without competition. If you can’t quarantine, at least buy from healthy stock and watch new fish closely. See do I need to quarantine new fish for more.
After they’re in
Add fish a few at a time, not all at once, so your filter isn’t overwhelmed — see can I add all my fish at once. Keep feeding light for a day or two, and check that tankmates are getting along.
The bottom line
Quarantine, acclimate slowly, net them in, and add in small groups. That routine spares your fish both shock and disease. Explore compatible companions in our species library.