Most tropical fish kept in freshwater aquariums today originate in rivers and lakes in Africa. Today, however, it is just as likely that the fish you buy are raised in a pond at a breeding facility.
The most important criteria for fish selection are size and compatibility. Most local fish stores are knowledgeable and can tell you which fish can be kept with which other fish, and which fish won't get along with your current tank's occupants. It's helpful to keep track of the fish you do have in the tank so you can consult this before buying a new specimen.
The other criterion is fish and tank size. Some fish, either because of size or personality, need a lot of space, and your local fish store can advise you here, too. As for the number and size of which that will fit your tank, the useful rule of thumb is one inch of fish per gallon of water.
Therefore, if the fish you are buying are an inch and a half long, you shouldn't put more than half a dozen in your 10-gallon tank, and that's assuming they are the only fish in the tank. If you already had 5 one-inch fish in the tank, and you add 3 of the 1.5-inch fish, you've pretty much filled your quota with that tank.
The solution? For most hobbyists, no more room for new fish means it's time to add another new tank! However, depending on our age, our spouses or parents may think otherwise!