The tropical fish that can live together are those that behave similarly and require similar water conditions. It is important to introduce your compatible fish to the tank one at a time so that you can observe any potential problems between them.
Fish are classified into three categories: peaceful, semi-aggressive and aggressive. Peaceful fish can always be kept in the same tank. If you choose to have multiple semi-aggressive fish in your tank, make sure to have a fair number of peaceful fish as well. Try to avoid any substantial size differences between the peaceful and semi-aggressive fish, as the semi-aggressive fish can bully the peaceful fish.
Aggressive fish cannot be mixed with peaceful fish. Sometimes aggressive and semi-aggressive fish will not do well together if there is a large size difference between them. No matter what fish you have in your aquarium, it is a good idea to provide the fish with areas that they can hide in, such as rock caves and large shells. That way, if one fish begins to behave aggressively, the other fish have a place to hide for a while.
Common peaceful fish species include danios, guppies and tetras. Some semi-aggressive species are gouramis, barbs, eels and loaches. Aggressive tropical fish include cichlids, oscars and plecostomus.