Cardinal Tetra

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 because until recently it was difficult to breed in captivity. However, many breeders are now producing the fish, in most cases one can determine if the cardinal tetra is bred or wild caught due to damaged fins on wild caught specimens. Normally aquarists prefer to buy tank bred fish but some Brazilian ichthyologists believe that fishkeepers should continue to support the sustainable Cardinal fishery of the Amazon basin, since thousands of people are employed in the region to source fish for the aquarium trade. The fear is that if the fishermen didn't catch Cardinals, they could turn their attentions to deforestation. This doesn't seem to pose an environmental concern (the fish is very common in the wild.)

The fish is also effectively an annual species and has a lifespan of just a single year in nature. It lives for several years in captivity.

An entire industry is in place in Barcelos on the banks of Brazil's Rio Negro in which the local population catches fish for the aquarium trade. The cardinal fishery here is highly valued by the local people who act as stewards for the environment. The local people do not become involved in potentially environmentally damaging activities, such as deforestation, because they can make a sustainable living from the fishery.

Perhaps due to their wild-caught origins, cardinal tetras tend to be somewhat delicate in captivity. In the wild, these fish inhabit extremely soft, acidic waters, but seem to be tolerant of harder, more alkaline water conditions; a greater concern is probably polluted tank water (including high nitrate levels.) They prefer warmer water temperatures (in the upper 70s F or warmer), and will readily accept most forms of dry food. Captive-bred cardinals tend to adapt better to hard water better than wild-caught cardinals.

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 R. Axelrod.

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 and fecundity. The preferred temperature range of the fish is 21°C to 28°C, and an aquarium containing these fishes should be well filtered, in order to remove assorted pollutants: in particular, a well established biological filter that controls ammonia, nitrites and nitrates is beneficial. If the intention of the aquarist is to breed the species in captivity, then the water chemistry of the aquarium water should match that of the wild habitat - filtration of the aquarium water over peat is one means of achieving this.

As the species is a shoaling species in the wild, groups of six or more individuals should be maintained in an aquarium. They will shoal with their close cousins neon tetras however, so a combination of these two species totalling at least six should suffice. The larger the numbers present in an aquarium (subject to the usual constraints imposed by space and filtration efficiency), the better, and large shoals in any case form an impressive and visually stunning display.

The species will feed upon a wide range of aquarium foods, though again, conditioning fishes of this species for breeding will usually require the use of live foods such as Daphnia.
Aquarium furnishings should be planned with some care. Live aquatic plants, as well as providing an additional biological filtration component that assists with nitrate management in the aquarium, provide an environment that resembles at least part of the wild habitat, and fine-leaved plants such as Cabomba are usually the plants of choice, though other plants such as Amazon Swordplants and Vallisneria are equally suitable for an aquarium housing the cardinal tetra. Floating plants providing shade will also be welcomed by the species: this is connected with the breeding of the fish, which will now be covered. A perfect biotope type would be lots of bogwood, a few native plants, with dark substrate and subdued lighting with floating plants. With this add 5+ Marbled Hatchets, 4+ Angels, 15+ Cardinals, 4 Ottos, Several Corys and a few nice plecs.

The species exists in a number of different colour forms or phenotypes. A "gold" and "silver-blonde" form exist in the Rio Negro drainage which have less blue in the longitudinal stripe. The normal form from the Rio Negro drainage has a blue stripe which extends to the adipose fin, while the Orinoco drainage phenotype has a stripe which stops posteriorly of the adipose. The Orinoco phenotype may represent a subspecies of P. axelrodi.