The black tetra (Gymnocorymbus ternetzi) also known as the black skirt tetra, high-fin black skirt tetra or black widow tetra is a freshwaterfish of the characin family (family Characidae) (which includes all tetras, including the piranha), of order Characiformes. It is native to the Paraguayand Guaporé River basins of southern Brazil, Argentina, and Bolivia.
Growing to about 6 cm (2.5 in) total length, the black tetra has a roughly tetragonal body shape and is greyish in colour, fading from light at the nose to near black at the tail. Two prominent black vertical bars appear just posterior of the gills. It is a schooling fish, and requires at least 6 in a group, but they can also make do with 3-4 tetras per aquarium.