Cardinal tetra (Paracheirodon axelrodi)

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. It is also often called the red neon tetra. Cheirodon axelrodi (the original name) and Hyphessobrycon cardinalis are obsolete synonyms.

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, and is named after the brightly red colored cardinal bird.

It was difficult to breed in captivity. However, many breeders are now producing the pet fish; in most cases one can determine if the cardinal tetra is bred or wild caught due to damaged fins on wild caught specimens.

Habitat
The origins of the cardinal tetra (blackwater rivers) is characterised by an acidic pH (5 -7), low mineral content (soft water) 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, with optimum pH at 5.8.

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. Tank currents can help encourage shoaling behavior.

Feeding
The pet fish 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.

Pet fish care
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 pets fish: this is connected with the breeding of the pet fish, which will now be covered. A perfect biotope to promote breeding, would be lots of bogwood, a few live native plants, with dark substrate and subdued lighting with floating plants.

Breeding in Captivity
The cardinal tetra, in the wild, swims upstream in large numbers to parts of its native river habitat that are completely enclosed above by rainforest canopy. Such waters are subject to heavy shading by the rainforest trees, and virtually no sunlight reaches them. Here, the fishes spawn in large aggregations. In the aquarium, a single pair can be conditioned for breeding, but the breeding aquarium not only needs to contain water with the correct chemical parameters cited above: the breeding aquarium needs to be heavily shaded to mimic the low light conditions of the fish's native spawning grounds. If the fishes are ready to spawn, the male, which will be the slimmer of the two fishes in outline, will pursue the female into fine-leaved plants: her fuller outline, which usually indicates the presence of ripe eggs within her reproductive tract, should be readily apparent at this point. If the female is ready, she will allow the male to swim alongside her, and together, the pair will release eggs and sperm.

Apart from the stringent requirements with respect to water chemistry, one of the major difficulties mitigating against success in captive breeding of the pet fish is the nature of the newly laid and fertilised eggs. The eggs of the cardinal tetra are photosensitive, and 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.

If the eggs are fertile, and kept in darkened surroundings, they will hatch in approximately 3 days at 28°C. Free swimming fry remain photosensitive for at least the first 7 days of life, and need to be introduced to increasing light levels on a gradual basis. During this time, they are approximately 4 mm in length, and require infusoria or liquid fry food. Newly hatched brine shrimp and other similar live foods such as sifted Daphnia can be fed to the growing fry at between 7 and 14 days of age. Growth continues at a modest rate, and the fishes assume full adult colouration only after a period of approximately 8 to 12 weeks, depending upon quality of food and aquarium water.