The 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 (1.25 in) total length, the cardinal tetra has the striking iridescent blue line characteristic of the Paracheirodon species laterally bisecting the fish, with the body below this line being bright red in color. 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, however.

The cardinal tetra is a very popular aquarium fish but is less widespread than the neon tetra as it is difficult to breed in captivity; most specimens sold in the aquarium trade must consequently be collected in the wild.

P. axelrodi is also often called the red neon tetra. Cheirodon axelrodi (the original name) and Hyphessobrycon cardinalis are obsolete synonyms. The fish's common name, cardinal tetra, refers to the brilliant red coloration, reminiscent of a cardinal's robes. The specific epithet honors ichthyologist Herbert Axelrod .

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