Once you have decided to start keeping fish, the next big decision is to choose fish. While it may seem fun to pick out one of each beautiful fish in the store, this is not likely to be successful.
The first factor to consider when buying fish is compatibility. This means, first of all, putting fish together only if they will get along. Fish which are subjected to aggression will be stressed and more likely to get sick. Speak to the shopkeeper about his fish. Buy fish only from a shopkeeper who seems knowledgeable. He should be able to advise you on compatibility.
All your fish must subsist in the same water conditions or you will not be able to provide an optimal habitat for them. Fast-swimming fish will disturb those that swim more slowly, and may take more than their share of the food. Fish with different food requirements are also more difficult to raise in one aquarium. Large fish may eat small fish if they fit in their mouths.
When choosing fish at a store, check that the fish are healthy. They should be active and crowd to the front when you approach the tank. Don’t buy fish with poor color, fins close to the body or strange swimming behavior. If one fish in the tank appears sick, don’t get any fish from that tank. If the tank is not clean or there are dead animals floating in it, go to a different store.
Some fish swim all over the tank while others tend to stay at a specific level. It’s a good idea to combine fish which swim at different levels, so that your whole tank will be inhabited.