How do you add new fish to your tank safely? Plan the quarantine, float and first batch
Bringing new fish home is the riskiest hour of the year for your tank. Sick newcomers can spread illness, sudden water changes stress fish, and too many arrivals at once overload the filter.
This guide gives you a planner that builds a step-by-step arrival plan from your tank, then explains quarantine, floating the bag, and adding a few fish at a time. Sources differ on quarantine length, so we say so. Ask a fish vet if a fish looks unwell.
You have a cycled tank, a happy shoal, and a shop bag with one more fish in it. The next hour matters more than it looks. New fish can bring illness with them, they get stressed by sudden changes in water, and every extra fish adds waste your filter has to handle. A little planning makes the difference.
Plan your new arrivals
Tell the planner about your tank and it builds a step-by-step plan: what to do on arrival day, how long to wait, and how many fish to add in each batch.
Go slowly: quarantine first, float the bag, add a few fish at a time, and test the water before the next batch.
Why quarantine is worth it
Quarantine means keeping new fish in a separate tank before they join your main one. Illness can show up days or weeks after you buy a fish, and a sick newcomer can pass it to every fish you already have. Guides commonly suggest at least four weeks, and some say up to eight or more. Sources differ, so treat four weeks as a minimum and watch the fish: eating well, swimming normally and no spots, white fluff or clamped fins. If a fish looks unwell, ask a vet who treats fish; don't guess at medicines.
No spare tank? Then be extra choosy about where you buy, watch the new fish closely for the first weeks, and add them in small groups.
Float, then mix: how to acclimatise
Acclimatising means helping a fish get used to your water's temperature and chemistry gently. A common method:
- Dim the aquarium light and float the sealed bag for about 20 minutes, so the temperatures even out.
- Open the bag and add a little tank water (around a tenth of the bag), then wait 5-10 minutes.
- Repeat every 5-10 minutes until the bag is about half tank water. That takes roughly 45-60 minutes in total.
- Net the fish into the tank. Don't pour the bag water in; it can carry waste and germs.
If your water and the shop's are very similar, some guides say 25-30 minutes is enough. Sensitive fish may need longer. Never rush it.
Why a few at a time
Your filter is home to helpful bacteria that turn toxic ammonia (from fish waste) into nitrite, and then into much less harmful nitrate. They only multiply as fast as the waste grows. Add many fish at once and the bacteria fall behind, so ammonia and nitrite climb. RSPCA advice is plain: don't add large numbers of fish at one time. After each batch, test ammonia and nitrite every day or two for the first week or so, and wait until both read zero before the next batch.
What to look for
- Ammonia and nitrite at zero before the first batch and before each new one.
- Fish that suit your tank size and each other: check the adult size and temperament, not the size in the shop.
- Bright, active fish with clear eyes, full fins and a healthy appetite.
- Regular partial water changes in the weeks after each batch.