A headstander is any of several species of South American fish, including Anostomus ternetzi, Anostomus anostomus (family Anostomidae) and members of genus Chilodus ^[1][2] from the family Chilodontidae. The name derives from their habit of swimming at a 45° angle, head pointed downwards, as if "standing on their heads".

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Headstanders are a group of freshwater fishes that live in streams of South America.^[1][2] Some species, such as Chilodus punctatus and C. gracilis, are common aquarium fishes as well.^[2] In nature, they are predominantly found in shallow streams with strong currents and a lot of algae, which they feed off of. They prefer slightly acidic water with medium hardness. The headstander will eat almost any kind of food, but mostly enjoy hair algae.^[3] Some headstanders can reach up to 12 cm (4 3/4 inches) in length. They tend to be very active, sensitive to shadows, and like to jump. They also have a tendency to be slightly aggressive. In aquaria, they are most peaceful when kept as a single specimens or in groups of more than 6.^[4]