There are a number of things to consider when buying tropical fish aquarium for sale. Choosing from the several hundred different breeds is certainly not an easy task. Even after 
making a choice, one is faced with the problem of knowing how healthy the fish are. I will share some information that will help in your decision making.
It is particularly important to consider the appearance of the fish you want to buy. Most people prefer brightly colored fish, and of course they should be healthy too. Stay away from fish that looks dark or pale in appearance. It is very likely that they are sick or distressed. They might not be able to survive the ordeal of being moved to a new tank.
You also need to observe how the fish behave as they swim around in their tank. Some fish will swim up to the glass, while others will swim away from you. It is better to choose the ones that do not swim away too hide. These fish might have problems that could get worse after you buy them. If possible, also watch they fish as they feed. Some fish will start eating as soon as the food is put in. Some of the others on the other hand will wait for the other fish to finish eating first.
The best tropical fish to buy are the ones that start eating immediately. They are more likely to be the healthiest fish in the tank. The ones that wait are naturally timid, and unable to compete. In the long run, these timid fish are less likely to survive.
Buying pregnant fish is popular amongst some fish buyers. It is a good way to increase fish population. It is not a good idea though, to buy fish that are almost ready to give birth. They could get very stressed by moving to a new environment, and could possibly die. The size of their bellies will give you an indication of how far advanced they are.
Ask lots of questions before buying tropical fish. Most reputable fish stores will be willing to answer your questions. Remember to ask which breeds live together well. That way you avoid the situation of ending up with fish that spend all their time fighting each other.