Cardinal tetras are small fish, males grow to about 2.5 cm in length in the wild, but can attain lengths of 5 cm in an aquarium. This species is characterized by a horizontal neon blue stripe and deep red markings. The neon stripe of Paracheirodon axelrodi becomes iridescent because of external lighting. As the angle of light that strikes them changes, the color of the neon stripe turns from green to blue or vice versa. During the night, when no light hits the stripe, cardinal tetras are a transparent, brownish color. Cardinal tetras, as well as many other tetras, have a small adipose fin on their dorsal side at the tail end of their bodies. Cardinal tetras have a single row of dentary teeth.
Sometimes cardinal tetras are confused with neon tetras, Paracheirodon innesi. Cardinal tetras, however, have a longer red band dorsally.
Paracheirodon axelrodi exhibit some sexual dimorphism. Females are slightly larger and wider than males. Females have larger stomachs as well. Mature male cardinal tetras have bony pelvic fin hooks.
Average mass: 0.13 g.
Range length: in wild - 2.5 cm, in aquarium - 5 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: female larger; sexes shaped differently