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Cardinal Tetras in their natural habitat
Paracheirodon axelrodi

By Ivan Mikolji

Paracheirodon axelrodi is one of the species featured in the Freshwater Natural Aquarium Documentary.

For more information, pictures, and videos of Paracheirodon axelrodi commonly called the Cardinal Tetras in the aquarium hobby click HERE.

Paracheirodon axelrodi is commonly known for the aquarium hobbyist as cardinal tetras which are one of the most popular tropical aquarium fish. Most people who have had a freshwater aquarium have probably owned one, or like in my case, in which at some time had five hundred of them schooling in a large aquarium. In my point of view, a nicely planted aquarium with a school of Paracheirodon axelrodi is probably one of the most beautiful freshwater aquarium layouts.

Every year hundreds of thousands of Paracheirodon axelrodi are caught in the wild from Colombia, Brazil and Venezuela then exported all around the world. Despite their popularity in the aquarium hobby little is probably known about their natural habitats and their behavior in the wild. I hope that the information and pictures in this article can guide the aquarium hobbyists which are interested in simulating the habitat where these wonderful fish come from in their aquariums. All this data can also help give everyone a wider comprehension or idea on how to breed them plus some photographing tips that can be useful in aquarium photography.

Cardinal Tetras habitat

In the wild, Paracheirodon axelrodi can be found in small, slow moving streams or creeks. These are always fairly clear to crystal clear having low silt content. I have taken some water parameters with the following readings… PH is always under 6.5 going as low as 5.5. The water temperature ranges from 26c. to 23c.

These creeks are normally shaded by the dense foliage of tall trees which maintain the water with a constant temperature, which is low, compared to the outside temperatures which are always in between 28c. and 33c. They also protect the water from evaporating and drying out in the dry season. The river bed and banks regularly consist of silica sand.

There are always aquatic or land plants present in their habitats. The Paracheirodon axelrodi use these flora to hide from potential predators. These aquatic and land plant leaves also act as tentacles which “trap” the organic matter that the current drags. This organic matter called Perifitum is what I have seen the Paracheirodon axelrodi feed off and it is consumed on a regular basis. The aquatic plants are always yellowish in color, and are only green where the sunlight can get through the tree vault, 30 to 40 feet up top.


Cardinal TetrasParacheirodon axelrodi

As some scarce sun rays enter the water they create a breathtaking lighting effect which I call the Lucanus effect named after Oliver Lucanus, who introduced me to underwater videoing, photography and made me aware of this lighting effect. The effect looks like the spot lights in a theater, lighting up a fish on stage. A good Lucanus effect picture is hard to achieve. You have to wait a long time until a fish makes up his mind to pass under the ray of light. Sometimes when the fish decides to pass, a cloud passes on top and the effect is lost. Sometimes the fish passes so fast under the ray that you don’t have the time to snap the picture. This starts the whole process of waiting for the fish and cloud passing again. This effect can probably be achieved in an aquarium by dimming the general light in the aquarium and adding one or more spot lights or flashlights aiming in the desired place where the picture wants to be taken. If your water is too clear for the ray to show you might choose to stir some of the gravel to make some small particles float which can reflect from the spotlights beam of light.

Paracheirodon axelrodi

The Paracheirodon axelrodi are very sociable with humans in the wild. You can get very close to them before they start swimming away. In some creeks, if you float immobile for 1 or 2 minutes, they start swimming next to you and sometimes start biting your skin. They have a certain preference in how your ears and lip skin tastes which sometimes makes it hard for me to concentrate on filming or photographing. You might think that a cardinal tetra wouldn’t bite hard, but once you have twenty or thirty of them biting you all over your body for a couple of hours, you actually start to feel them bite harder and harder each time. I always start to laugh, thinking they should have been taxonomically categorized as Piranhas!


There are two seasons in the southern Venezuelan Orinoquia (Orinoco River basin), the dry and rainy season. Every year the rivers and creeks in this area follow a pattern that influences the life and habitat of the Paracheirodon axelrodi. In the dry season, which normally starts in October or November, these creeks water volume and levels get to their lowest states and narrow down from 20 feet wide to 4 feet wide. They also go from 6 feet deep to a pretty constant depth of 3 feet.

During this time of the year the Paracheirodon axelrodi are already medium sized which is 2 to 2.5 cm. long. As the rivers become narrow due to the lack of water volume, they get confined to the main canal of the stream. Here you can find dense schools of them.

Paracheirodon axelrodi habitatCardinal Tetras habitat

I have noticed that at this time of the year they become bottom dwelling fish. As the rivers current cannot drag all the leaves and branches that fall in the water, the river bed gets covered in a rug of leaf litter from the trees above. The Paracheirodon axelrodi make this their dry season home. You can see them covering the river bottom and not schooling at all, they seem static like, as if they were in a power saving mode. The river bottom becomes a display which looks like sprinkled glitter on a rug of brown leaves. Most of their predators like the Monocirrhus polyacanthus (leaf fish) which normally hunts in the water’s surface have to change their habits and move to the bottom of the river to hunt them. Other fish which normally school with Paracheirodon axelrodi like the Hemigrammus stictus (red-base tetra) do not change their habits and stay mid to top-water dwellers. Other predators of the Paracheirodon axelrodi include young Cichla (Peacock bass), Hoplerythrinus unitaeniatus (golden wolf fish), Potamorrhaphis eigenmanni (needlefish), and the always present Hoplias malabaricus (wolf fish). By the end of the dry season just when it starts to rain most of the Paracheirodon axelrodi are already 3 cm.+ or Jumbo size.

In the rainy season, the creeks start to grow with the abundant rain. In the rainy season, the creeks start to grow due to the abundant rain. Waters normally averaging 3 feet in depth grow to an average of 9 feet deep.

Paracheirodon axelrodi commonly called the Cardinal Tetras in their natural habitatParacheirodon axelrodi











The currents get stronger and the creeks overflow into the jungle. All the leaf litter is dragged away by the currents, leaving the silica sand in display. This is the breeding season of the Paracheirodon axelrodi. At this moment most of the Paracheirodon axelrodi are jumbo sized, ranging from 3 to 3.5 cm. long. They become mid-water dwellers and start to school. They leave the rivers main canal bed and head towards the shores or swim in between the branches and flooded vegetation. At this time of the year they don’t come out to the open.

Paracheirodon axelrodi

The schools of Paracheirodon axelrodi make brief pit stops in places where “they” can see their food, pick on the organic matter stuck on the plant leaves, and 15 seconds later keep on schooling until they make a next stop to feed 15 feet away. During the day, the schools of Paracheirodon axelrodi are regularly small, ranging from five to fifteen individuals. In the late afternoon, one or two hours before dusk these small schools start meeting and growing in individuals, with some schools getting as large as forty to fifty of them.

Cardinal Tetras habitatParacheirodon axelrodi habitat

Paracheirodon axelrodi are truly the fish that stick out the most in the wild. They do not try to imitate or blend into their natural surroundings at all. Even being so small, you can spot them from more than 20 feet away when you are snorkeling and much farther away if the sun hits them. In my opinion, they are the color kings of the wild.


For more information, pictures, and videos on Paracheirodon axelrodi click HERE.

This article was published on the February 2009, Tropical Fish Hobbyist Magazine (TFH) issue.








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