Minimum Tank Size: 20 Gallons (75 Liters)
Care Level: Easy
Water Conditions: PH 4.5-7.0 and Soft
Temperature: 75-84°F (24-29 °C)
Maximum Size: 2 inches (5 cm)
The cardinal tetra (Paracheirodon axelrodi) is a small, beautiful fresh water fish. It closely resembles the neon tetra, and is often mistaken for this more common fish. Cardinal tetras are native to Brazil and Venezuela, and are found in the upper Orinoco and Negro rivers.
Most cardinal tetras available in fish stores are wild-caught, though more captive-bred ones have become available in recent years. However, unlike many other fish, the cardinal tetra fishery is believed to be sustainable and this species is not currently threatened in the wild. A good way to tell if the fish are wild-caught, is to inspect them for fin damage caused by the nets used to catch them.
Cardinal tetras primarily inhabits small, heavily shaded streams and the waters are often a tangled maze of tree roots, branches and plant debris. However, it can also be found in more open waters, but only in areas with dense aquatic plant cover.
They are very short-lived in the wild, and it is believed that most only have a lifespan of around one year. In the home aquarium, they will live longer, and if provided with the proper conditions, they may live up to three years. However, their life span will be significantly reduced if they are kept in poor conditions, or in hard water.
Cardinal tetras tend to be delicate in captivity, and are very sensitive to poor water conditions (including nitrates). Because of this, they should be provided with the largest aquarium your available space (and budget) can accomodate. While many people recommend a 10 gallon (38 liter) tank as the minimum tank size for them, a 20 gallon (75 liter) would actually be a better choice.
It is very important that they be provided with soft, acidic water in their tank. While captive bred cardinal tetras are somewhat adaptable to harder waters, wild-caught ones don’t adapt well to basic, moderately hard water.
Cardinal tetras are a shoaling fish, and they should always be kept in groups of at least six. With that being said, they do prefer larger groups, and many people recommend keeping them in groups of no less than ten.
They should be kept in subdued lighting, and this can best be accomplished through the use of floating plants or a dim light. A dark substrate is best, though a sand substrate is also acceptable. Some aquarists cover the substrate with leaf litter to more closely mimic their natural environment. But this should not be viewed as a requirement, and brings a whole host of other problems with it.
However, live aquatic plants are an absolute must, and some excellent plants which can thrive in low light are cryptocoryne wendtii, Java ferns, Java moss, and most species of Anubias.
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Cardinal tetras are primarily carnivorous in the wild, and they feed on tiny invertebrates, insect larvae and insect eggs. However, even wild-caught neon tetras will quickly take to a flake food, though they will still appreciate the addition of live or frozen foods to their diets.
When choosing live foods, the best choices are daphnia, mosquito larvae (illegal to cultivate in some areas), and adult brine shrimp. Many other commonly available live foods tend to be too large for cardinal tetras to consume, though you could also try blackworms or bloodworms to see if your fish will accept them.
Breeding cardinal tetras can be difficult, and they require very precise conditions to spawn. In the wild, they breed in areas that are almost completely enclosed by the rainforest canopy. Under the darkness of the intertwined tree branches, they breed in massive shoals, though they will breed in small groups, or pairs in the home aquarium.
A breeding tank must mimic their natural breeding habitat as closely as possible, and should be completely shaded from any light. The water must also be very soft and acidic, and nitrates must be kept to an absolute minimum. The ideal temperature for a breeding tank is around 82ºF (28ºC).
Any fish tank used for breeding should be completely cycled, and a sponge filter is recommended for filtration. Live plants should also be provided, as this is where the cardinal tetras will scatter their eggs. Java moss is an excellent choice for this, and will also provide cover for newborn fry.
Once a male and a female have been selected for breeding, they should be placed in separate tanks. They should then be conditioned with live foods, until the female appears swollen with eggs, and the male is displaying vivid colors. Once they look ready to spawn, they should be placed together in the breeding tank.
If they are ready to breed, the male will pursue the female into the live plants. If the female is receptive, she will swim alongside the male, and the male will release his sperm, while the female simultaneously releases her eggs.
The parents can be removed once the eggs become visible, or they can be left in the aquarium until you notice free swimming fry. However, the fry should be raised on their own, and the adults should be removed as soon as it can be confirmed that breeding has taken place.
If the eggs are not exposed to any light, they will hatch in two to three days. If the eggs are exposed to any sort of illumination, they will die. The newborn fry are also sensitive to light, and should be kept in low light conditions for at least a week after hatching. At this point they can be slowly introduced to increasing levels of light.
Newly hatched cardinal tetra fry can be fed infusoria or any of the commercially available fry foods. After a few weeks, they can be moved on to baby brine shrimp, micro worms, or finely crushed flake food.
How to Tell Neon Tetras and Cardinal Tetras Apart
If you’re having trouble telling neon tetras and cardinal tetras apart, you’re not alone. But there are several ways that you can distinguish these two very similar looking fish. The best way to tell them apart is to look at the red stripe on their body. On a neon tetra it only extends halfway to its nose, while a cardinal tetra’s extends almost the entire length of its body. Also, the blue stripe will be much more vivid on a cardinal tetra, although this takes a seasoned eye to see.