Do your research.

It pays to know what types of fish will thrive in the type of aquarium environment that you have recreated. Try to read as much about different fish varieties that thrive in different environments. You should know how to differentiate between fish that do well on a temperate environment and those species that thrive in a tropical aquarium setting. Try to choose fish varieties that will be able to live comfortably in your type of aquarium.

Know how to determine a healthy fish.

Try to learn as much about the signs that show how healthy a fish is. This will ensure that you buy aquarium fish that won't end up dead in your tank or infect other fish in your tank. Try also to learn the signs of fish that are stressed out. Most marine fish usually turn colors when they are stressed. Some fish like tangs, triggers and even damsels may either darken or pale their colors when they are stressed.

Other signals can be exhibited by the fish themselves. Aquarium fish that act listless or hides often may be a sign that there may be problem. Try also looking for actively swimming fish with no torn fins or discolored scales. Missing fins and discolored scales may be a sign of some kind of infection.

Try to observe a fish variety for some time before you buy them.

You can always put your chosen fish variety under observation for a time by paying a deposit at a fish pet store and ask them to hold it for you for a week or so. You can always say that you need to have your tank set up before you can get the fish from them. Not only will this help you observe how the fish will be reacting and behaving, you may also be giving them time to get readjusted to their environment.

The most colorful fish that you might find in the pet store might usually be fresh from transport from some other place, which might put the fish in some form of stress. Giving it some time to readjust will help keep them in shape so that you don't have to worry about getting unhealthy fish in your aquarium.