Introduce the fish carefully. Put them in a plastic bag and let it float for at least one hour. This will make sure that their temperature becomes equal to that of the tank. Change the water slowly. It is quite possible that the water composition where the fish was earlier was totally different from the water in your tank. Keep on adding small amounts of water from the tank. Do this very slowly over a period of half an hour or maybe more than that. This activity is very important for the health of the fish. By this, the differences in chemicals and dissolved solids will be removed and kidneys and gills of the fish will be protected.

Many people do not provide the settling period for the tank and put the fish straight on. Such people suffer from heavy losses, become disheartened and give up. Some people like an amateur aquarist may keep some cheaper fishes till he gains experience. Thus it's better to gain proper knowledge before indulging in this hobby.