Cardinal tetra, Paracheirodon axelrodi

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The Cardinal tetra is one of the world's most popular aquarium fish species. We remind you exactly why…

What are Cardinal tetras and where do they come from?

This tiny characin comes from the soft, acidic waters of the Rio Negro drainage in Brazil and reaches just a couple of centimetres in length. It’s a gregarious species that appreciates being in a large group and makes for a stunning aquarium showpiece.

It comes from the middle to upper Rio Negro basin and the Orinoco basin.

What sort of habitats are they found in?

Cardinals are found in flooded forest areas, particularly small, shallow creeks which lie alongside larger rivers and tributaries. Like other fishes, Cardinals enter when the rivers swell and feed, breed and take refuge within.

When the rivers initially flood the tetras move upstream and laterally into the forest, but return as waters recede in the dry season. The waters in which they live therefore vary according to time of year, but at the peak of the dry season they could be swimming in just a couple of inches of water in a rapidly evaporating creek.

What do they feed on in the wild?

According to Instituto Nacional de Pesquisas da Amazonia, 2004, natural diet consists of very small crustaceans, mesofauna, eggs, algae, detritus, and some other types of prey.

Most small crustaceans eaten are cladocera (small, spherical moinids, daphnids, and macrothricids), as well as some copepods (benthonic Harpacticidae). Rotifera and Thecamoebae are included in the mesofauna they eat.

The types of algae they take include unicellular diatoms (Navicularia and Pinnularia) and some green algae (Chlamydomonas, Conjugatophyta and Volvocaceae). They sometimes feed on dead fish, eating the detritus of their muscular, proteinaceous and membraneous tissues.

Paracheirodon axelrodi may also eat ants, fly larvae or pupae, mites, newly-hatched shrimp, fungus, fruit, and fish larvae. They feed constantly, swimming through a rich soup of tiny foods.

In the aquarium Cardinal tetras feed readily on flake foods, granulated and small pellets, and many frozen and live foods including bloodworm, brineshrimp, black mosquito larvae and Daphnia. Cyclops and marine zooplankton can also be offered to them.

How many species are there?

Three are described from the genus Paracheirodon. The Cardinal tetra (P. axelrodi) from the Rio Negro from Brazil to Colombia, the Neon tetra (P. innesi) from the western Amazon in Peru and the far west of Brazil’s Amazonas state — and the Green neon tetra (P. simulans) also from the Rio Negro in Brazil.

There may be undescribed species and Heiko Bleher discovered a potentially new species in a creek in the Purus river basin, south of the Amazon.

What other species do they live alongside?

Some claim that Cardinals are also found alongside the Green neon tetra, When PFK visited the Rio Negro, Hockey stick pencilfish (Nannostomus eques) and Black winged hatchetfish (Carnegiella marthae) were abundant and likely to share some habitat with Cardinal tetras.

Heiko Bleher also set up an authentic Cardinal tetra biotope at an exhibition, including Rummynose tetras (Hemigrammus bleheri) and in PFK’s December 2009 issue, while writing about Rummynose, listed Otocinclus, other pencilfish and hatchetfish, Corydoras, Rineloricaria, Ancistrus and Peckoltia brevis — so there’s plenty of choice.

Are those in the UK wild-caught or captive-bred?

Most on sale here are captive-bred, but wild-caught fish are also available. The Cardinal is Brazil’s most economically important ornamental fish species and millions are exported from the Rio Negro area every year.

The Barcelos region of Brazil has been widely studied by scientists who have helped create a controlled sustainable fishery for Cardinal tetras and other aquarium fishes.

How long do they live?

In the wild Cardinals are believed to live for around a year. During the dry season billions become stranded in shallow pools and creeks and are often consumed by predators or die when the pool dries completely. In captivity they’ll live for around five years if conditions are right.

Why are they so colourful?

The Cardinal has bright metallic stripes that you might think would make it rather obvious to predators. However, research has shown the opposite is true.

The colours change in response to lighting and background conditions. Although they look bright in colourless clear water, their stripes appear darker when viewed through very dark tannin-stained water in the wild.

Their metallic stripes reflect light only at a specific angle and when near the surface this reflects a bright mirror image onto the underside of the water surface. This provides a false target for predators while the fish remains less conspicuous.

Are there any different colour forms?

There appear to be two different morphs, one from the Rio Negro and one from the Rio Orinoco. Characiform experts Dr Stanley Weitzman and Dr William Fink studied them and found they differed little in morphology, fin ray or scale counts, but did have slightly different markings.

The variety from the Colombian drainages of the Rio Orinoco have a shorter blue stripe which does not meet the adipose fin, while the Rio Negro form has a longer blue stripe that does reach.

There is also a morph with a golden stripe and one with a silver stripe instead of a blue one. Breeders in the Czech Republic have also fixed a golden form.

Do they suffer from any ailments in the aquarium?

In well-kept aquariums Cardinals are hardy, easy to keep and generally not disease prone. However, newly-imported fish are susceptible to Neon tetra disease caused by a tiny parasite called Pleistophora hyphessobryconis.

This can be difficult to eradicate and it’s important to ensure fish are free of it before buying. Beware fish that are less active and have pale patches.

What size tank do they need?

As they are a shoaling fish, keep them in large groups, so a 45cm/18” aquarium would be the minimum. They look far better as very large shoals in much larger aquariums. Invest in a shoal of 50 plus if you have a big enough tank.

Are they seen as good community fish?

Cardinals are ideal community fish and do not bother other fishes. However, they are quite small so ensure any other fish kept with them are not going to try and eat them. Angels, in particular, can often predate on Cardinals, so be careful if mixing.

How would I set up the perfect biotope?

The authentic Rio Negro habitat of the Cardinal is blackwater, 28°C/82°F with very low pH and zero hardness. The water is made acidic and tannin-stained by leaf litter and tree branches with a pH around 4. Few plants grow in this habitat due to extreme acidity, low light from the black water and there are no nutrients for aquatic plants to use as fertiliser.

Substrate is fine sand or mud, with leaves and wood.

Achieve black water by using soft water, like RO, then filtering through peat, proprietary blackwater extract, or bogwood and beech, oak or almond leaves.  

For such a small prey fish typically inhabiting shallow water with little or no current, make sure filtration is adequate but gentle.

Are Cardinals difficult to breed?

In the wild they spawn in the rainy season when the habitat becomes very much larger, more foods becomes available and these small fish can disperse throughout the flooded forest. Temperatures will also drop.

In the aquarium they are likely to spawn frequently, but the eggs are often eaten by tank mates, including catfish.

The eggs are light sensitive too, so heavy planting or lily leaves, floating plants and bogwood will be necessary to shade them in the community tank. In a separate tank set up for breeding, the eggs can be shaded by covering the tank with newspaper.

What happens when the fish spawn?

The eggs become fertilised by the milt of nearby males. Mating takes place at twilight with the male embracing the female while swimming and fertilisation is external. Males fertilise as the female scatters some 500 eggs.

Once laid the eggs sink and stick to plants. Wool spawning mops can be used in a breeding tank.

What will trigger them to spawn?

To breed Cardinals successfully in captivity, pH should be between 5.5 and 6, the temperature should be at 24°C/75°F and lighting should be dim.

How should I raise the fry?

Parents should be removed after spawning as they will eat the eggs and fry. Eggs hatch within 24-30 hours of fertilisation and fry are independent within three to four days of hatching.

At this point begin to offer some regular feeds of liquid fry food, infusoria, or rotifers.

Larger fry will thrive on small amounts of food processed hard-boiled egg yolk. Powdered eggs will also work and then powdered fry food.

Males and females reach sexual maturity at some nine months.

This item first appeared in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.