A dash of red here, a streak of blue there and a bit of black thrown in; tetras are known for their colors and their quick, darting movements. A staple in almost any freshwater community tank, they can be a joy to watch with their sometimes boisterous behavior.
Tetras are schooling fish, meaning they prefer to be in a small to large group of the same species. Before deciding what other tank mates to add, make sure you have at least a small school -- six minimum -- of these small fish together. Not schooling could cause unwarranted stress on your fish, which could potentially lead to death or disruptive behavior in the tank. Like many other schooling fish, you'll only see their true behaviors and most vivid color patterns when they're kept in a school.
In many cases, adding another small school of another species of tetra can work well in your community tank. With all the different types to choose from, don't worry about getting bored by having only tetras. For example, the boldly colored blue neon and red neon tetras can offset the more blandly colored red-eye characin. While most tetras are slim-bodied, you can also add a little variety with a small school of bleeding heart tetras.
In general, tetras do well with similarly sized and similarly active community fish of different species. Many barbs and danios, although not all, make suitable tank mates for a tetra-oriented community tank. Rasboros, cherry barbs or rosy barbs are three prime examples of companion fish for a tetra community. Some bottom dwellers, such as cory cats or certain loaches, can also add to the variety in your tank without causing stressful environments for your tetra schools. Species that aren't particularly active, like dwarf cichlids, could become stressed out by the sometimes quick movements of some tetras, but their peaceful behavior lends well to a tetra community.
Unsurprisingly, more than a few species don't take kindly to sharing their space with tetras. Larger, aggressive species such as oscars might make a quick meal of your tetras or terrorize them throughout the tank. Most cichlids are a big no-no due to their territoriality and aggressiveness. Bottom line: stay away from anything noticeably larger and more aggressive.