Cardinal Tetra

About the species

The cardinal tetra is regularly confused with the plain neon tetra in shops, but they're two distinct species: the cardinal is slightly larger, its red band runs the full length of the body rather than stopping halfway, and it comes from far softer, more acidic water than its more common relative. It's a shoaling fish from the blackwater rivers of the Rio Negro and upper Orinoco, thriving in naturally soft water while gradually declining in hard tap water, even if it survives there in the short term. Given soft, stable water and a proper-sized shoal, it's one of the most vividly coloured and peaceable schooling fish in the hobby.

Cardinal tetra or neon tetra?

The key difference is how far the red extends: on a cardinal tetra the red band runs the full length of the body, while on a neon tetra (Paracheirodon innesi) it only covers the rear half. The cardinal is also slightly larger, reaching around 3cm standard length as an adult, compared with the smaller neon. Neon tetras today are almost entirely farm-raised, which makes them more tolerant of tap water, whereas cardinals sold in shops are still often wild-caught from the Rio Negro basin, tying them more closely to the soft, acidic water they come from.

A native of the Amazon's blackwater

The cardinal tetra comes from the Rio Negro and upper Orinoco drainages of Brazil, Colombia, and Venezuela, where it lives in so-called blackwater – river channels stained brown to amber by tannins released from decaying leaf litter in flooded forest. That water is extremely soft and acidic, with pH sometimes dropping below 5. This isn't a minor detail: it's the reason a cardinal tetra fares worse over the long term in hard, alkaline tap water, even though it can survive there for a while.

A fishery that protects the river instead of emptying it

According to Project Piaba, a long-running research initiative on the Rio Negro, the cardinal tetra is one of the most abundant vertebrate species in the Amazon and makes up around 80% of the artisanal catch of ornamental fish taken for the aquarium trade; despite an annual harvest of roughly 20 million fish, no population decline has been detected. The trade generates income the project estimates at over two million US dollars a year for riverside communities, creating an economic incentive to protect the flooded forest rather than clear it for pasture.

Why ordinary tap water usually isn't enough

Cardinal tetras need soft, acidic water with a pH of roughly 4.0 to 6.5, general hardness of 5 to 12°dH, and carbonate hardness of 1 to 4°dH, at a temperature of 23 to 27°C. They're sensitive to sudden shifts in water chemistry, so they should go into a well-established, stable tank rather than a newly cycled one. Hard, alkaline tap water isn't necessarily fatal outright, but a cardinal kept in it tends to lose colour over time and becomes more prone to illness.

A shoal of at least ten

Cardinal tetras are strongly shoaling fish that turn skittish, pale, and stressed in too small a group. A group of at least 8 to 10 is recommended; in a smaller shoal, the fish shows neither its natural behaviour nor its full colour intensity. A larger shoal in a densely planted tank with subdued, diffuse lighting best mirrors its natural blackwater habitat.

Living alongside other soft-water species

The best tankmates are other calm species from similarly soft water: smaller tetras, harlequin rasboras, corydoras catfish, and peaceful dwarf cichlids that tolerate the same soft, slightly acidic conditions. Avoid fast, boisterous, or territorial species that would push a more timid shoal into a corner of the tank, and avoid species that need hard, alkaline water — a water-chemistry compromise eventually costs one species or the other.

Diet

The cardinal tetra is omnivorous, hunting tiny worms and crustaceans in the wild; in the aquarium it accepts good-quality small-grain pellets or flakes, supplemented with frozen or live foods such as brine shrimp and daphnia. Its small mouth needs correspondingly small food — oversized pellets or flakes just foul the water rather than getting eaten.

Common mistakes

The most common mistake is expecting a cardinal tetra to be as undemanding as a neon tetra and adding it straight to hard tap water with no preparation. A second common mistake is too small a shoal, and a third is adding it to a newly set-up tank before parameters have stabilised. Because these fish are often still wild-caught, a longer quarantine period before mixing them with other fish is advisable.

Pros & cons