danio, any of several slender tropical fishes of the genera Danio and Brachydanio in the carp family, Cyprinidae. Danios are hardy and swim actively about in schools. They are generally some 4–5 cm (1.5–2 inches) long. Several are often kept in home aquariums. Among these are the zebra danio, or zebra fish (B. rerio), a popular species with lengthwise blue and yellow stripes, and the giant danio (D. malabaricus), a striped blue and yellow fish about 11 cm (4 inches) long.