De kweek van Kardinaaltetras

Breeding the Cardinal Tetra

Breeding the Cardinal Tetra is not easy. There are undoubtedly several ways to breed this colorful fish, but this is how I do it from experience.

The preparation

Biotope aquarium with Cardinal Tetras
Biotope aquarium with Cardinal Tetras

To get a good breeding group, it is best to purchase about 20 Cardinal Tetras that are as young as possible. Make sure there is an aquarium ready for your breeding group. The water must have a low KH. The pH should be slightly acidic at pH 6. The water may contain few dissolved salts and therefore have a low conductivity of max. 100 μS. The temperature in my fish room is around 23 degrees Celsius.

To keep the water values stable, I used aquasoil (without plant food) as a soil substrate, which has a stabilizing effect on the pH and KH. Using this method of preparation, I have had several successes in breeding, resulting in schools of between 100 and 150 fish.

However, due to an unfortunate turn of events, I unfortunately lost my breeding stock 2 years ago. The breeding report described is of about 20 fish that were not prepared in the manner described, but which I was offered half-grown and which had actually been in water that was too hard and too alkaline for too long. The breeding resulted in a mediocre result of 35 fry, which is good for a new breeding strain.

Feed the breeding group with mainly live food. For example, give them freshly hatched artemia nauplii, grindal worms, enchytrae, etc. See also this link for an explanation of cultivating fry food (in Dutch).

Grindalwormen 3 002 1
Breeding the Cardinal Tetra 12

Conditioning the Cardinal Tetra

In the young fish the difference between males and females is not visible. When they start to become sexually mature it slowly becomes visible. The females grow slightly larger than the males. The females are also slightly plumper in shape than the slimmer males.

One to two weeks before you start breeding, put the males and females separately in their own aquarium. The females now have the opportunity to save eggs so that they will soon be full of eggs for breeding. By separating the sexes, the attraction they feel for each other is also stronger when they get back together.


To make water in which you can breed the Cardinal Tetra, I first make a peat extract myself. For this I use a bucket with 10 liters of demineralized water. I add about a liter of peat to the demineralized water. I aerate the water for a few days until the peat extract has a conductivity of about 60 μS.

The next step is to make the breeding water using even more demineralized water. I make the demineralized water myself with a cation/anion ion exchanger (see this link with an explanation about the ion exchanger (in Dutch)). The water from the ion exchanger has a conductivity of approximately 1.6μS. Water from most reverse osmosis devices do not achieve such a low conductivity. The water in the Cardinal Tetra’s habitat has a conductivity of approximately 8μS.

Now take 20 liters of demineralized water and add 0.4 liters of peat extract of 60 μS. If the peat extract is not that strong (say 30 μS), add, for example, 0.8 liters of peat extract.

This brings the pH of the breeding water to approximately pH 5.0. Measure the pH with a drop test or other color indicator. Water with such a low conductivity has no buffering capacity (kh=0) and a very low ion concentration. As a result, a pH probe gives a very unstable value.
I personally use a solution of methyl red as a pH indicator (color should be orange-red for pH5).
Obviously the precision of this measurement is not that great, but my experience is that for this type of water it works better as a pH probe. Reading the color is a matter of experience.

By diluting the peat extract, the conductivity is around 4 μS and the pH of the breeding water is approximately pH 5.0.

The breeding tanks

Breeding tray with slanted egg grid
Breeding tray with slanted egg grid

To increase the chance of a successful breeding, I use two breeding tanks. The breeding tanks have a capacity of 25 liters. I placed an egg grid just above the bottom. When laying the eggs, they fall down through the grid. The parents cannot get through the grid. This ensures that the parents cannot eat the eggs.

I only place a 25 Watt heating element in the breeding tanks. The temperature in my fish room is 23° Celsius, so the temperature in the breeding tank can be raised to 28° Celsius in a few hours.
The breeding tanks do not contain regular or plastic plants.

The egg grid fits exactly the length of the breeding container. It becomes slightly larger in depth. The egg grid is therefore slightly inclined so that you can look underneath it.

Because the eggs are light sensitive, but also because cardinal tetras mate in the dark (usually at night), I darken the containers. I only leave a gap of about 2 centimeters in the center of the tank.

Breeding tank with 2 centimeter slit in the aquarium cover.
Breeding tank with 2 centimeter slit in the aquarium cover.

Rinse all attributes with demineralized water, the breeding tanks, the egg grids and the heater. You do this to keep the conductivity of the water (in μS) as low as possible.

The spawn

Make sure that the breeding water is the same temperature as the water in which the breeding group and separate males and females swim. Take 5 males and 5 females and place them in one of the breeding tanks. Use the drip method to get them used to the lower pH and lower conductivity in the breeding tanks. Repeat this for the second breeding tank. Turn on the heating (In my fish room the standard temperature is 23° Celsius) and let the temperature slowly rise to 28° Celsius.

Now check every morning whether eggs have been laid. To do this, remove the bottom half of the blackout. Look with a light under the grid to see if you can see any eggs. If so, there are always bad eggs, the bad eggs turn white and are the easiest to recognize. The good, fertilized eggs are transparent and difficult to see.

If you find eggs, return the parents to their group aquarium using the drip method. If there are no eggs after five days, place the parents back in the group aquarium using the drip method.

The eggs and raising the young Cardinal Tetras

After removing the parents, I transfer the eggs to a five-liter tank. I can now use the breeding tank again for another spawn. The young fish are therefore more concentrated in the breeding tank. This allows me to feed and monitor them better. Save the remaining water from the breeding tank. You’ll need this later.

First I remove the bad white eggs to prevent them from contaminating the good eggs.
I do this by stirring the container vigorously in a circle along the circumference with a spoon. The good eggs sink quickly and due to the rotation they concentrate in the middle at the bottom. The bad eggs float longer. I then transfer the good eggs to the 5-liter tank with an air tube. Black out the five-liter tank completely.

Breeding tank
Breeding tank

4th day: On the 4th day after laying the eggs, remove the blackout. The eggs have hatched after about 24 to 36 hours, but the larvae still feed on a yolk sac and do not yet need to be fed. That yolk sac is now starting to run out

Very small pinheads are visible at the bottom of the breeding tank, which respond to light. Add an air tube and aerate very little, this will provide some movement and oxygen in the water.

5th day: On the fifth day you start feeding. Although most young Neon Tetras can handle newly hatched brine shrimp larvae as their first food, these are just too large for young Cardinal Tetras. Therefore, first give them pure Paramecium and/or vinegar eels twice a day for a few days.

6th day: On the sixth day you can feed them with vinegar eels (twice a day). Also try giving them some newly hatched brine shrimp. After 20 minutes, check whether the young Cardinal Tetras have red bellies from the brine shrimp. This is a sign that they have eaten well. If they do not yet have red bellies, the brine shrimp nauplii are still too large. Then feed them with vinegar eels for another day.

7th day: On the seventh day you feed them brine shrimp (twice a day). From today on, you can also give them artemia in the morning and micro nematodes in the evening.

You have now been working on this spawn for about 2 weeks. Time for some maintenance. Carefully siphon off the bottom and remove any accumulated dirt. Top up the water with the same water left over from the breeding tank.

14th day: From the 14th day after spawning, the bottom can be carefully cleaned (siphoned) twice a week. Now also slowly increase the conductivity. You increase the conductivity by using a little more tap water. The pH of the water is now less important. The young Cardinal Tetras are now better able to withstand this.

4 to 5 weeks: After 4 to 5 weeks the blue color starts to become visible. (photo below 29 December)

From now on they eat brine shrimp in the morning and grindal worms in the evening. I sometimes have some large ones among the small grindal worms, which I siphon away later.

8 weeks: After 8 weeks I transfer the fry to a 25 liter tank. This time unfortunately a poor result in terms of numbers. Only 35 young are produced from this breeding. I put them together with 38 equally sized and approximately equally old Firehead Tetra (Hemigrammus bleheri). I breed this species in the same way as the Cardinal Tetras.

From now on I will feed them three times a day. They get brine shrimp in the morning, grindal worms in the afternoon and dust food in the evening.


As you can see, breeding the Cardinal Tetra requires a bit more effort than some other species. However, with the right preparation and knowledge of water values, it is doable.

Author and photos

Piet van Amelsvoort – Aquariumvereniging De Kempvis


John de Lange – AquaInfo

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