The cardinal tetra (Paracheirodon axelrodi) is a freshwater fish of the characin family. It is native to the upper Orinoco and Negro Rivers in South America. Growing to about 1.2 inches 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 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. Some ichthyologists believe fishkeepers should continue to support the sustainable cardinal fishery of the Amazon basin, since thousands of people are employed in the region to capture fish for the aquarium trade. If those fishermen lost their livelihoods catching cardinals and other tropical fish, they might turn their attention to engaging in deforestation.
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.
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 may 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 [75 °F or warmer], and will readily accept most forms of dry food. Captive-bred cardinals tend to adapt to hard water better than wild-caught cardinals.
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 79 to 82 °F. However, if necessary they will live at 75 °F. 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 is the use of reverse osmosis water.
As the species is a shoaling species in the wild, groups of six or more individuals should be maintained in an aquarium although bigger groups are preferred. They will shoal with their close cousins neon tetras, though, so a combination of these two species totalling at least six should suffice (again, larger groups are preferred). Tank currents can help encourage shoaling behavior. 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 on 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.
Apart from the stringent requirements with respect to water chemistry, one of the major difficulties in captive breeding of the species is the photosensitivity of the eggs; they will die if exposed to bright light. Consequently, after spawning, the fishes should be removed and the aquarium covered to darken it, thus providing the developing eggs with the conditions necessary for development.
Aquarium furnishings should be planned with some care. Live aquatic plants, as well as providing additional biological filtration components to assist with nitrate management, 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 them. Floating plants providing shade will also be welcomed by the species; this is connected with the breeding of the fish. A perfect biotope to promote breeding would be bogwood, a few live native plants, dark substrate and subdued lighting with floating plants.