Cardinal Tetra

Scientific name: Paracheirodon axelrodi

Adult size: about 3.5 cm

Origin: Rio Negro and Orinoco basins, Brazil, Colombia and Venezuela

Water: 23–29 °C, pH 3.5–7.5, 1–12 dGH

Group: 8–10 or more

Diet: omnivore, small dried, frozen and live foods

Temperament: peaceful, schooling

The cardinal tetra looks like a neon at first glance, but the red band runs the full length of the body beneath the blue stripe, rather than stopping halfway. Adults reach about 3.5 cm, a little larger than a neon. It comes from blackwater rivers of the Rio Negro and Orinoco basins, where the water is very soft, very acidic and stained brown by tannins, the compounds that leach from decaying leaves and wood. Wild fish prefer those conditions, though tank-raised stock accepts a much wider range. It is warmer-loving than a neon, and is generally kept at the upper end of tropical temperatures. Cardinals are schooling fish and settle only in a group; eight to ten is a realistic minimum and a larger, mixed-sex group behaves more naturally. They are peaceful and share a tank well with other small, calm species. In the wild they eat small invertebrates, crustaceans and filamentous algae.

Water: 23–29 °C and pH 3.5–7.5, hardness 1–12 dGH. Soft, acidic and warm is the target, and stability matters more than exact numbers. Add them only to a cycled, mature tank, meaning the filter has established the bacteria that break down fish waste.

Tank and planting: planted, with dark substrate, driftwood and leaf litter for tannins and subdued light. Leave open water in the middle for the group to move through.

Feeding: quality small flake or granule as the staple, plus regular frozen or live daphnia, bloodworm and mosquito larvae for colour and condition.