Color widow tetra The black tetra (Gymnocorymbus ternetzi), also known as the black skirt tetra, petticoat tetra, high-fin black skirt tetra, black widow tetra and blackamoor, is a freshwater fish of the characin family (Characidae). It is native to the Paraguay River basin of south-central Brazil (mainly Pantanal region), Paraguay and northeast Argentina, but there are also populations in the upper Paraná and Paraíba do Sul Rivers that likely were introduced.[1] It was formerly reported from the Guapore River, but this population is part of G. flaviolimai, which is found throughout the Madeira River basin and was described in 2015.[1] The black tetra is often kept in aquariums.[2]