The neon tetra (Paracheirodon innesi) is a freshwater fish of the characin family (family Characidae) of order Characiformes. The type species of its genus, it is native to blackwater and clearwater streams in the Orinoco and Amazon basin of South America. Its bright colouring makes the fish visible to conspecifics in the dark blackwater streams, and is also the main reason for its popularity among tropical fish hobbyists.
The neon tetra has a light-blue back over a silver-white abdomen. The fish is characterized by an iridescent blue horizontal stripe along each side of the fish from its nose to the base of the adipose fin, and an iridescent red stripe that begins at the middle of the body and extends posteriorly to the base of the caudal fin. Rarely, they develop a dark olive-green sheen lining on their backs. The fish is partially transparent (including fins) except for these markings. During the night, the blue and red become gray or black as the fish rests—it reactivates once it becomes active in the morning. It grows to approximately 3 cm (1.2 in) in overall length. Sexual dimorphism is slight, with the female having a slightly larger belly, and a bent iridescent stripe rather than the male's straight stripe. They have recently become available in a long-fin variety.