Budding aquarists often begin their tanks with some type of tetra (Characins). Tetras are very hardy fish that get along with several different types of community fish, although some can be a bit fin nippy (Serpae Tetras for example). These fish have a tendency to become stressed when paired with fast moving fishes or fish that dart around constantly. Tetras will get along with most community fish of a similar size. The one strict rule of freshwater aquariums is if a fish can fit in another fishes mouth, then eventually it will fit in that fishes mouth.