Breeding the Cardinal Tetra

From Microcosm Aquarium Explorer


Breeding your schools of Cardinals is challenging but possible.

Breed these beauties with confidence and know-how

By Mary E. Sweeney The Cardinal Tetra (Paracheirodon axelrodi) is one of the best known of the aquarium fishes. This is partly because they are so photogenic and their likenesses appear frequently in aquarium books and magazines.

One catch: They can be frustrating to keep and breed. In fact, these lovely little fishes can be much more difficult to keep over the long term than that other tough-to-please fish, the Discus. Cardinals are frequently kept with Discus—provided the Cardinals are large enough not to be food for the Discus. Few people would argue that a planted tank with a number of Discus and a substantial school of Cardinal Tetras is not a breathtaking sight.

Cardinal Tetras in Nature

Cardinal tetras are natives of northern South America, and found in the quiet blackwater tributaries of the Orinoco and Rio Negro rivers. They are among the most populous of the small tetras, so there is absolutely no fertility problem involved with these fish in the wild. If fact, they are found by the millions, and most of the specimens sold are wild-caught during the dry season when they are easy to capture. Fortunately, this has no negative impact on the current wild population, as it would otherwise be snack food for piranhas. In the wild, cardinals live on worms and small crustaceans.

The cardinal tetra is the most striking of the Paracheirodon species. The fins are clear, but their little 1.5- to 2-inch forms are incredibly colorful, with primarily red bodies bisected laterally by an iridescent blue stripe running from the eye to the tail. When the fish are relaxed and in good form, these colors are intense, but they bleach out when kept poorly or under stress. Sexually mature females are stockier than males and are generally a little larger.

Keeping Cardinals Well

Keep cardinal tetras in a school. Most references recommend at least six fish in the school, but 10 or a dozen fish are even better. The more cardinals that can be kept together, the healthier and more secure they are. Buy the smallest cardinals you can find so that you have more control over their diet and grow-out conditions. The more certain you can be that they have been kept on a proper diet and in proper water conditions throughout their development, the better your chances of having success with breeding the fish. It’s bad enough to have the disappointment of working toward spawning a fish without success, but to invest the time and effort without ever having had a chance is very sad indeed. So, do your best to find adolescents and rear them yourself on high-quality (and preferably live) foods.

The water should be 82 degrees Fahrenheit, below 6.0 pH and soft. Lighting should be subdued. Natural driftwood and peat filtration will help to keep the water softened and acidified.

Avoid plants in the grow-out tank. Plants will not thrive in the low light, and old and dying leaves contribute to poor water quality. (One of my house rules is that if I am growing plants, they are the priority, and if I am growing fish, the plants are plastic.) Do use blackwater extract, oakleaf tea and other natural sources of tannic and humic acid to condition the water. These products are quite effective but should not be overdone.

Cleanliness is very important in keeping cardinals, so unless you simply must have a deep substrate, it is useful to have a very shallow and dark (even black) substrate that is easily kept clean. A bare-bottom tank with the external sides, bottom, and back painted black or another dark color would be appropriate. A dense “growth” of plastic plants is a good egg-saver, and it may even be possible for the young and adults to live together in such an arrangement (I’m not sure about this, but it’s worth a try if that’s the situation).

Preparation and Water Conditions

Even in the community aquarium, mature female cardinals will be quite apparent over time. They fill with eggs, and their little bellies look ready to burst. If you are planning to spawn them as pairs in gallon containers, you will need to pick out the females.

Possible male.
Well-rounded specimen, likely female.

When you find ripe females, capture them, and keep them in a separate tank so that you can identify and condition them. You don’t want them to go too long without spawning, or they could become eggbound; but if you are going to keep them as pairs, you need to be able to sex them. It would be very frustrating if you missed your window of opportunity because you failed to segregate the females. The primary requirement for breeding cardinal tetras is very soft, clean water with high levels of humic and tannic acids (from peat or blackwater extract). I did not get any viable eggs until I reduced the conductivity of the water to 40 to 80 microSemens (discus produce viable eggs at 80 microSemens). The eggs are quite sensitive to light, as these are blackwater fish.

You can breed your cardinals in a species tank or by pairs. Either way, you can get fry. It just depends how many you feel you can raise. If you are not blessed with naturally soft water, you can use distilled water, reverse osmosis water, rain water, or snow melt water that is filtered over peat and aged for several days. A small amount of blackwater extract is no harm, either. The blackwater extracts will bring out color and can trigger spawning; use them carefully and according to directions.

Change the chemical composition of your water very carefully, as well. Regular water changes usually help. The pH should be about 4.5 to 5.5 (incidentally, this is the pH at which I have had the most success in breeding discus – not 6.0 to 6.5, which is often recommended. At pH this low, there is no ammonia in the water). The temperature should be 80 to 82 degrees Fahrenheit. The water should be shallow, no deeper than 5 or 6 inches. Don’t forget to age your water!

It is critical that the water be free of bacteria. Some breeders use potassium permanganate to sterilize bare tanks before introducing the conditioned cardinals. I have not found this to be necessary yet, but I would have no problem with using this or even erythromycin if bacteria became a problem and I was losing eggs consistently. However, I would wait until there was a problem before taking these steps. Usually, I tend to go the other way and prefer an aquarium that has been set up a long time. Healthy aquariums that use sponge filtration and are left empty, but running for a few weeks are marvelous for almost any use.


You will need to protect the eggs from being eaten. Use a spawning mop or a fine grate elevated slightly from the bottom of the tank. Sterilized peat moss is also used by some breeders as a substrate to protect the eggs and acidify the water. If you want to use peat as a substrate, soak it in water until it sinks. The water in the spawning tank should only be about 4 inches deep. The bubbles from the sponge filter will not disturb the water at this depth, as they would not be breaking under the water. The spawning tank should have still water. Whether you choose to spawn pairs or groups, you need to condition the adults well. Conditioning the breeders is not done in the spawning tank. There should be no food given in the spawning tank to keep the water as pure as possible for the eggs and fry. While cardinals will take flake food without complaint, for conditioning, live and frozen foods are ideal. Live white worms and bloodworms, enriched brine shrimp, and Daphnia will bring the fish into condition in one or two weeks at the most. Frozen natural foods are also good, but not as good as food that needs to be chased. The hunt for live food seems to have an aphrodisiac effect on fishes.

When the fish have spawned, which should happen within about five days, remove the adults from the spawning tank so that the eggs and fry can develop unmolested. When you shine a light on the bottom of the tank, you will see the fry scooting around the bottom. The fry are very, very tiny (small children seem to have the most success in spying the newly hatched fry).

Feeding the Fry

In five or six days, the yolk sac will disappear, and it will be time to feed the fry. This is a critical stage when the whole project can go south because of starvation, or feeding too late, too soon or too much. The development of the eggs and fry are temperature dependent to a degree, and keeping the water at the recommended temperature will keep things moving along nicely. Cooler water temperature slows the development of the eggs, absorption of the yolk sac, digestion of food and growth of the fry in general.

Paramecium, infusoria or one of the liquid fry products should be fed very sparingly. Generally, I know when to feed very small fry when I see them pecking at the glass and the sponge filter. That’s the value of using an old but clean aquarium with an old filter; these have a population of first foods for fry, and if you can see the fry feeding from these, you know you can start to add green water or other suitable foods. Baby Brine Shrimp are fed to the fry from the time they will accept them until the shrimp are outgrown. You know the fry are eating the brine shrimp when their little bellies turn orange after you have added a small amount of brine shrimp.

For most people, feeding the fry seems to be the most difficult part of raising Cardinal Tetras. Adding a few small pond snails to the tank after the fry have hatched will help a bit with housekeeping, but careful feeding is still necessary.

Feed your fry at least twice a day in the smallest amount imaginable, only a drop or two with an eyedropper. After two or three days, begin to change out very small amounts of water, and add back water of the same chemistry and temperature. While changing water when the fry are still minute, use an airstone on the end of a length of airline tubing to prevent the fry from being siphoned out of the water.

Cardinal Tetra fry grow very slowly and do not start to color up until they are about 8 or 10 or more weeks of age. They are sexually mature at 6 to 8 months of age. Though they only live for a year or so in the wild, and could even be considered annual fish, they have been known to live for six or seven years in captivity when the fishkeeper acquires a “cardinal rule” and learns to keep them well.

Further reading:

Image credit: JJ
Text credit: MES