The Cardinal tetra (Paracheirodon axelrodi) is almost as famous as its close relative the Neon tetra, but much less suitable for beginner aquarists. A Cardinal tetra is more sensitive to high levels of soluble waste and most of the specimens available in pet shops are wild caught, since it is very difficult to coax Cardinal tetras into breeding in aquariums. A beginner aquarist can still successfully keep Cardinal tetras, but you must be willing to monitor the water quality and keep the levels of waste down. High levels of nitrite are especially dangerous for Cardinal tetras. The water should preferably be acidic and really soft. Strive to keep the hardness under dGH 4 and the pH value between 4.6 and 6.2. You can force Cardinal tetras to adapt to harder water and higher pH-values, but Cardinal tetras living in such an aquarium will be extra sensitive and more prone to illness. Keep the water temperature between 73 and 81 degrees F (23 and 27 degrees C).
Cardinal tetras and Neon tetras are often mixed up, since they are both blue and red tetras. Some pet shops will even market their Cardinal tetras as “Red Neons” even though they are two entirely different species and actually very easy to tell apart. Both Neons and Cardinals have a horizontal iridescent blue line that bisects the body. Under this line you will find a red stripe, and it is this stripe that makes it so simple to tell the two species apart. If the red stripe stops halfway to the nose, the fish is a Neon tetra. If the red stripe continues much longer, the fish is instead a Cardinal tetra. To put it simple: the Cardinal tetra has more red colouration than the Neon tetra. The abundance of red pigment is actually what gave the Cardinal tetra its name; the long red stripe was thought to resemble the long red robe of a cardinal. The scientific name for this species, Paracheirodon axelrodi, is derived from the name of a renowned ichthyologist (fish expert) named Herbert Axelrod.
The Cardinal tetra works well in a community aquarium, even in a small one since this fish is peaceful and rarely grows larger than 2 inches. It is a schooling species and you should therefore keep at least 10 Cardinal tetras together in the aquarium if you want your fish to thrive. If you try to keep only one Cardinal tetra, it will most likely spend most of its time hiding and the stress will make it less resilient towards disease. Keeping a single tetra is also less visually appealing than keeping a large and healthy school of many tetras. Try to mimic natural environment for your Cardinals and include plenty of plants and other suitable hiding spots in the aquarium set up. Floating plants are recommended. There should also be an open area left for swimming.
Wild Cardinal tetras live in South America, in the northern Negro and Orinoco rivers. Their natural habitat has soft and acidic water. Even though this fish is hard to breed in captivity, it is highly prolific in its native environment and not considered an endangered species, despite the fact that a myriad of Cardinal tetras are captured every year for the aquarium trade. Wild Cardinal tetras rarely grow older than one year, so they have to reproduce quickly after being hatched. In an aquarium without any predators, a Cardinal tetra can grow older and live for several years.