Tetra

The word tetra describes various species of small fish in the order Characiformes.

Tetras include many species commonly kept in aquaria, such as neon tetras (pictured), phantom tetras, emperor tetras, ornate tetras, diamond tetras, and ornate tetras in the Characidae family, African tetras such as Congo tetras in the family Alestidae, and pencilfishes (Lebiasinidae).

Tetras are characterized by their adipose fin, a small, fleshy fin with no fin rays or other supporting structures, whose purpose is currently unknown, though some tetras lack the appendage. Most have a compressed and sometimes deep body. Some have unique adaptations, such as the Mexican cave fish which lacks eyes, and some of their relatives such as piranhas and pacus have strong jaws for biting plants and flesh, and the freshwater hatchetfishes, whose deep bodies and large pectoral fins allow them to glide above the water.