What to Know About Cardinal Tetra

Medically Reviewed by Vanesa Farmer, DVM on December 10, 2022
6 min read

The cardinal tetra is one of the most well-known aquarium fish. They're the world's most-in-demand Amazonian ornamental fish, accounting for almost 80% of the total fish export from Amazonas, a state in Brazil. These tetras are very popular among fish hobbyists for their peaceful nature and striking looks. They're quite hardy and easy to raise, making them the perfect fish for your aquarium, even if you're a beginning aquarist.  

The cardinal tetra is popular worldwide as an aquarium fish. They are the most economically valuable ornamental fish of the Amazon state. They're mostly found in Colombia, Brazil, and Venezuela in the mid-Rio Negro and upper Orinoco basins. Moreover, the highest number of fish (21%) in the middle Rio Negro basin region belong to this tetra species.

The scientific name of the cardinal tetra is Paracheirodon axelrodi. In different parts of the world, they're also called by these names:

  • Red neon
  • Neon tetra
  • Roter neon
  • Scarlet characin 

These fish are not considered threatened, as their populations are thriving and large enough in number. But many conservation experts fear that their large-scale export — amounting to 12 to 15 million annually — may lead to their extinction. This is why several projects have been launched focusing on the sustainable exploitation of the cardinal tetra.

Cardinal tetras mostly belong to tropical freshwater. These fish are not known to migrate but stay in the open waters of rivers, streams, shoals, and forest lakes. During the months of heavy rain, you can also find them in the blackwater-flooded forests of the Amazon basin called "igapo".

Cardinal tetras live in waters with specific speeds and depths. Such biotopes include:

  • Standing or slow-moving clear waters.
  • Water in the middle layer of shoals. 
  • Litter-covered beds of rivers and streams
  • Forest floor during the time of flooding.
  • Shaded waters with dense tree cover.
  • Shallow waters with water plants, penetrated by sunlight.

As the groups of cardinal tetra move between water plants and over submerged leaves and litter, they feed on phytoplankton — microscopic algae.

Cardinal tetras are counted among some of the most beautiful aquarium fish. Their most striking feature is the fluorescent blue stripe running laterally throughout their body. You can also find vibrant red markings running below the blue line. Even their belly and back are an attractive silver color, which adds to the beauty of the fish. Like other tetras, these fish also have small dorsal fins near their tails.  However, these fins have no color. 

The colors on cardinal tetra change with the angle of light falling on them. For example, the fish appear brown and transparent at night when no light falls on their horizontal stripes. But in the presence of external light, the stripe becomes iridescent. Moreover, by focusing the light at different angles, you'll find their neon stripe changes colors from blue to neon and vice versa. Since neon tetras also have similar color patterns on their bodies, many confuse these fish.

The size of cardinal tetras depends on the sex of the fish. The male fish attain a length of 2.5 cm in the wild but can grow up to 5 cm in a fish tank. In comparison, the females are slightly bigger and wider, with more-rounded stomachs.

To properly care for cardinal tetra, pay attention to these aspects:

Size of the aquarium. This is one of the most important criteria for deciding the well-being of your fish. Whether you buy young or adult fish, always focus on the adults while comparing their size with that of your fish tank.

Remember that some fish grow significantly, sometimes even double or triple their size from the time of purchase. If your current aquarium is not big enough to easily house such adults, it may create discomfort and stress for all the fish. Eventually, this could make them sick.

There's a simple formula to check if your pet fish will be suitable according to your aquarium size. It is:

Consider 1 liter of water for every centimeter of the adult fish.

This means a 5-centimeter-long adult cardinal tetra would require a tank holding at least 5 liters of water. This water content excludes the plants and other aquarium decorations you might add to your tank. 

According to such calculations, experts suggest that the ideal aquarium for cardinal tetras should be 60 cm or more in length.

The number of fishes. Whether you should get a single, a pair, or a group of fish will depend on the way of life of your fish species. 

In their natural habitat, cardinal tetras live in schools, including tens of thousands of fish. So, you need to add more than one fish to be able to mimic their natural environment. 

Some recommend that these tetras be kept in groups of five or more, so they feel safe. However, others suggest keeping less than 10 fish together in the aquarium might trigger stress and sickness in the cardinal tetras. 

Conditions in the aquarium. Every fish requires specific environmental conditions to survive and thrive in an aquarium. Among them, the most important conditions are:

  • Temperature: The right temperature for cardinal tetras is between 23°C (73.4°F) and 27°C (80.6°F) since they most often live in tropical waters. Some studies show they can even bear high temperatures around 33.3ºC (91.9°F). However, their survival rate falls sharply at temperatures below 19.6ºC (67.3°F).
  • pH: It has been seen that neons and other South American fish prefer a low ph because many live in acidic environments like blackwater streams. This means your aquarium's water should be slightly acidic with a pH between 4.0 and 6.0. However, some studies show that cardinal tetras can tolerate a wide range of pH, falling between 4.0 and 8.5. It's best to avoid the extreme pHs and stick to the medium values since many of these studies have been done only on the wild varieties.

Susceptibility to diseases and changing conditions. Cardinal tetras are prone to many parasitic infections like those caused by ich. Studies show many common ectoparasite species can be found on Amazonian fish like tetras. Make sure you know the symptoms of these parasitic infections and regularly check if they have occurred in your fish.

These tetras also become sick easily due to changes in the water quality. Research has found that probiotics made of nutrients, vitamins, and bacteria like Bacillus subtilis and Bacillus licheniformes can improve the water quality and boost the immune system of cardinal tetras.

Cardinal tetras are omnivorous and eat a wide variety of foods. This is why you should include different kinds of foods in their diet. 

In the wild, they eat eggs, algae, small crustaceans, marine worms, dead fish, and detritus — a kind of waste or debris. They may also feed on fish larvae, ants, fungus, newly hatched shrimp, mites, and pieces of fruit. 

In the diet you provide, add varieties of flake foods, freshly hatched brine shrimp, living edibles like Daphnia, frozen or live bloodworms, as well as other processed foods.

If you have any other fish in your aquarium, consider if they'll be compatible with the new cardinal tetras. For example, avoid putting the tetras together with larger fish like angelfish since they may prey on the tetras. Also, make sure that the diet of your existing fish matches the diet of the new tetras. 

Keep in mind that the lifespan of cardinal tetras is longer in captivity. While they may live for a year in the wild, they can survive for 5 or more years in a fish tank. So, be sure you can commit these many years to the care and protection of your pet fish.