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.
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.
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.
Acclimatising means helping a fish get used to your water's temperature and chemistry gently. A common method:
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.
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.