The Cardinal tetra (Paracheirodon axelrodi) is similar to more frequently kept Neon tetra, but is much more difficult to breed in aquariums. Since both species look similar to each other at first glance, they are sometimes mixed up with each other and the Cardinal tetra is sometimes erroneously referred to as "red neon tetra".

Telling them apart is however not very difficult. Both species have a characteristic sparkling blue line that bisects the body, and under this line you will notice a red lateral stripe. If this red coloration extends only halfway to the nose of the fish, you know that it is a Neon tetra. If the red coloration instead extends much longer, you are looking at a Cardinal tetra.

The red coloration of the Cardinal tetra was thought to resemble the long red robes worn by cardinals, hence the name. The scientific name of this species (Paracheirodon axelrodi) was given to it in honour of Herbert Axelrod, a highly regarded ichthyologist.

Since the Cardinal is quite difficult to breed in aquariums, a majority of the Cardinals in the aquarium trade have been wild caught. The native habitat of the Cardinal tetra is the upper Orinoco and Negro rivers in South America, where the water is acidic and very soft. Fortunately enough, the Cardinal tetra is very prolific in the wild and is not considered an endangered species.

It is only reluctant to breed when kept in aquariums. In the wild, it is uncommon for a Cardinal tetra to grow older than one year. When you keep Cardinal tetras in aquariums without any predators around, you can however make them survive for several years.

The Cardinal can be kept in community aquariums with other peaceful species that appreciates the same water conditions. It will usually stay smaller than 2 inches in length and a group of Cardinal tetras do not need a large aquarium to do well. This species is rarely found in beginner aquariums since it is quite scarce in the aquarium trade, but it is not overly sensitive and a dedicated beginner aquarist that is prepared to monitor the water chemistry and perform frequent water changes can usually make his or her Cardinal tetras thrive.

It is especially important to keep down the level of nitrate. The Cardinal tetra is a schooling fish and keeping at least ten specimens is recommended, since this will make the fish less shy and stressed. Cardinal tetras are also much more beautiful to watch when they form a big school, and living in a school makes them display a much broader variety of natural behaviours.

When you set up an aquarium for your Cardinal tetras you should ideally try to make it similar to the native habitat of the fish. A well planted aquarium that contains floating species is recommended, but you should also leave an area open for swimming. The water should be acidic and very soft. Keep the pH in the 4.6-6.2 range and the dGH under 4. Cardinal tetras can adapt to harder water and even alkaline conditions, but they will be much more sensitive and prone to illness.