1. Check out the health conditions of the fish carefully. If you are observant enough, you can check out both visible and intrinsic health conditions of the fish. First, you must observe the body of the fish. Pay attention to the scales and the fins. The scales should be shiny and smooth, while the fins should not have any cuts or nicks. If there is some fuzzy growth on the body of the fish, it is better left alone. Also check the areas under the eyes for any telltale health signs. Some fish would not 'look' ill, but they might have internal conditions. You can tell that if a fish is either too sluggish and spends most time at the bottom of the tank, or if it is hyperactive and keeps darting everywhere in the tank in a sort of frenzy.
2. You must next check the nature of the fish and its compatibility with the other fish in the tank. Do not select fish that fight a lot with other fish in the tank. They will be a pain to maintain as they will injure themselves often, and you will not be able to place other fish in the tank with them. Also, check out if the fish becomes more docile with another fish of its own species in the tank, preferably of the other gender.
3. Ask the stocker about the size the fish can grow to. You must consider the size of your aquarium here. The equation to maintain is one inch of fish size per gallon of aquarium space. You must not only consider the present size of the fish, but you must see what size it will attain when it becomes an adult.
4. Fish are temperamental little creatures, and most of their temperament depends on the temperature of water they are kept in. You must ask the stocker about the natural living conditions of the fish, as to whether it is tropical water or temperate water fish. That will give you an idea what kind of temperature you will have to maintain in your tank.
5. Finally, you must choose fish according to the food they eat. Different fish have different food habits and some of them are very finicky when it comes to eating. You must make sure that the food your fish wants is readily available, and that it is not going to be very expensive. You will also need to keep food for your fish always stocked.
These are the five factors on which you can base your shopping for fish. By adopting these factors, there are very less chances that you can go wrong in your selection of fish.
