Tropical fish belong to the ghost knifefish family (Apteronotidae). They originate in freshwater habitats in South America where ranging from Venezuela to the Paraguay–Paraná River, including the Amazon Basin. They are popular in aquaria. The fish is all black except for two white rings on its tail, and a white blaze on its nose, which can occasionally extend into a stripe down its back. It moves mainly by undulating a long fin on its underside. It will grow to a maximum length of 50 cm (20 in). It does not have scales.