Cardinal Tetra (Paracheirodon axelrodi) is a freshwater fish of the characin family (family Characidae) of order Characiformes. It is native to the upper Orinoco and Negro Rivers in South America. Growing to about 3 cm (1.2 in) total length, the cardinal tetra has the striking iridescent blue line characteristic of the genus Paracheirodon laterally bisecting the fish, with the body below this line being vivid red in color, hence the name "cardinal tetra". The cardinal tetra's appearance is similar to that of the closely related neon tetra, with which it is often confused; the neon's red coloration extends only about halfway to the nose, and the neon's blue stripe is a less vibrant blue.
The cardinal tetra forages in areas of slow-moving shallow water. It is predominantly predatory, generally feeding on tiny animals they find on underwater plants, roots and leaf litter. Creatures commonly eaten include the larvae of chironomid midges and microcrustaceans such as water fleas (Cladocera) of the families Moinidae, Macrotrichidae and Daphniidae, and Copepods of the family Harpacticidae. Other organisms eaten include other fly larvae, insect eggs, rotifers and testate amoebae.Given the origins of the cardinal tetra, namely blackwater rivers whose chemistry is characterised by an acidic pH, low mineral content and the presence of humic acids, the species is adaptable to a wide range of conditions in captivity, though deviation from the soft, acidic water chemistry of their native range will impact severely upon breeding, fecundity, and life expectancy. The preferred temperature range of the fish is 26 to 28 °C (79 to 82 °F). However, if necessary they will live at 24 °C (75 °F).