Nandidae (knifefish): predators in the aquarium – breeding experiences

Nandidae (spiny catfish) live in the tropics of Asia, Africa, and South America. They are predators that primarily feed on live fish, and some species also eat insect larvae, invertebrates, and aquatic crustaceans. I have kept 4 species.

Polycentrus schomburgki (Schomburgk's spiny catfish)

Context and Inspiration

On this fish, MUDr. Vladko Bydžovský wrote a detailed article on this platform. He kept these spiny catfish in the mid-1980s. Thus, at the beginning of my "aquaristic life." Due to a lack of experience, I relied on literature for keeping spiny catfish.

Preparation of the tank and spawning

From the common tank, I caught a breeding pair into an approximately 30-liter tank. Equipped only with a sponge filter, a heater, and I leaned a slate plate against the tank wall. The pair spawned on it, similarly to angelfish. The female laid eggs, and the male fertilized them. After several mating acts, the spawning was completed. I caught the pair.

Care of the eggs and methodological note

To the clutch of about 100 eggs, I placed an air stone. This supplied oxygenated water to the eggs. All other information was provided in his article by MUDr. Vladko Bydžovský. I write this so that aquarists realize that it is possible to keep and breed the same species in different ways.

Nandus nandus (large-mouthed spiny catfish)

Origin, acquisition of the first specimens, and identification of the fish

I obtained these fish from my unfortunately already deceased friend Bogusław Ziarka during a business trip to Warsaw. It was in 1993. In his small shop (in Polish - sklep), there was always some nice surprise waiting for me. I bought the last 3 fish. Luck was on my side - it was one male and 2 females.

These fish live in India, Thailand, and Myanmar (Burma). They can grow up to 20 cm. It is therefore possible that the fish I purchased were smaller and a very similar species Nandus nebulosus. I am led to this assumption by the fact that the fish I kept grew to a size of: the male just under 8 cm and the females were smaller.

I fed them live food. Fish, insect larvae, daphnia, black mosquito larvae, live brine shrimp. The fish practically did not accept crustaceans of the genus Cyclops, probably due to their small size. At this time, I was very busy with business, and therefore my observations of the fish were very irregular.

Observations of spawning and eggs

The fish spawn in open water. The first spawning occurred without my attention. I noticed small grayish balls on the bottom of the tank and assumed they were decaying dead Cyclops.

I siphoned the tank and did not feed it this food anymore. The water temperature was 26 °C. After returning from a several-day trip, I noticed the same spotted bottom. I did not feed them Cyclops, and then with the help of a magnifying glass, I found out that they were eggs. The fish did not pay any attention to them.

Development and predatory behavior of the fry

The next day, part of the eggs hatched. The hatched fry looked like small dark brown triangles. Upon examining them with a magnifying glass, I found that the front part of their bodies is dark, and the back part and fins are completely transparent. The next day, more eggs hatched. Later, I discovered what is typical for some spiny catfish. They spawn with all the females that are in the tank.

After three days, the fry began to move, although it cannot be called swimming. The transparent part of their body and fins slide along the bottom. After another 5 days, their movement accelerated. I started feeding them. At first, I gave them brine shrimp nauplii. They easily accepted even strained small plankton. They hunt it just like adult fish. They move close to the food. To a distance where they can seize the food. By opening their mouths, they suck in their prey. They rest for a moment and then move to the next morsel. It appears almost comical. A fish just under two millimeters long behaves very similarly to an adult individual. Even from the very beginning of their life, they are predators.

Polycentropsis abbreviata (African spiny catfish)

Origin, description, and sexual differences

Another spiny catfish that I kept is Polycentropsis abbreviata (African spiny catfish). It is distributed in West Africa, Niger, Lagos, and Ogowe. In nature, it grows up to 8 cm. In an aquarium, it is smaller. My fish measured: males 6 cm and the females are smaller. The sexual differences are well noticeable. The male has a robust head and a high forehead. The females are smaller, and the belly is noticeably more rounded. The water temperature for spawning is 26 °C - 28 °C.

Conditions for keeping and food

Food: live fish, daphnia, brine shrimp, fewer black mosquito larvae, and brine shrimp. I kept the fish in a tank measuring 80 x 30 x 25 cm (L x W x H). Sand substrate, sponge filter, and remnants of coconut shell remained on the bottom from the ancistrus.

Spawning and construction of the nest

The first spawning took place in this coconut shell. Only three fish swam away. It was similar with the next spawning.

After this experience, I placed a plate on the surface. The male immediately occupied it. Under the plate, the male "pumps" air bubbles. He does not do it like cichlids or other labyrinth fish. He positions himself diagonally under the plate. With his mouth, he takes in air, and bubbles flow under the plate until it is completely filled. After a short display, he embraces the female. He turns her belly up. The fertilized eggs rise into the bubbles. He then covers the eggs with bubbles and follows with further spawning.

Reflections on the unsuccessful breeding at the bottom

There are not many eggs. I have never recorded more than 30 fry swimming away. I pondered for a long time the reason why there were so few fish from the spawning in the coconut shell.

I suspect that there was little oxygen at the bottom. There were no bubbles in the shell. There could also have been a higher water pressure at the bottom than at the surface. Raising fry with small food is easier. Despite all the care, some of the fry always died. I do not know the reason.

Afonandus sheljuzhkoj

From Africa, Afonandus sheljuzhkoj is also known from the Ivory Coast and Benin. It grows to a size of 5 cm. It was imported to Austria in the 1960s. The breeding was unsuccessful.

Monocirrhus polyacanthus (one-whisker spiny catfish)

Geographical distribution and size

Another species in my tanks was Monocirrhus polyacanthus (one-whisker spiny catfish). This species originates from the Amazon basin, Rio Negro, and western Guyana.

It is necessary to provide live fish and daphnia. The fish did not accept other food. It is probably the most well-known spiny catfish. It grows to a size of 8 cm.

First attempt at breeding (1992)

I attempted to breed these fish twice. The first time in 1992. The fish imported from the Netherlands were in very poor condition. Of the thirteen fish purchased, 7 were dead. Of the remaining 6, none started to accept food and soon died. The last fish lived about a year.

Characteristics of appearance and camouflage

During the observation of its behavior, I noticed that this species most resembles a decaying leaf floating in the water current among all the spiny catfish . It seems to move more like sliding than swimming in the tank. It often changes color. It slowly moves towards the offered fish while lying on the bottom. If it gets within catching distance, it suddenly extends and opens its mouth. The resulting suction draws the fish in.

Spiny catfish are capable of swallowing prey up to two-thirds of their body size. If a part of the fish is larger for some time, it gradually disappears into the fish.

I mostly fed guppies Poecilia sphenops. Aquarists call them Black Molly. It is interesting that they were first accepted when they were light-colored, later speckled, and completely black ones were rejected . Eventually, I caught them. I do not know the reason for this behavior. Apparently, the black fish were not considered suitable food. Completely black fish sometimes have small spots of black pigment among their scales, and it is possible that this was recognized by the fish, which is why the black fish were not consumed. Perhaps in time, some scientist will provide an answer to this.

Second attempt at breeding

I made the second attempt by purchasing a group of fish from a Czech wholesaler. Initially, they prospered well, but over time, one fish after another died until I was left with nothing.

From the magazine Der Macropode, I know that they spawn similarly to angelfish on the leaves of plants or objects in the aquarium, and out of necessity, also on the glass of the aquarium. The male cares for the eggs.

Spiny catfish do not have red, gold, or other bright colors. Their body is colored in various shades of brown, from light coffee to very dark, sometimes almost black. However, the observation of these color changes is very interesting.

Food and (in)appropriate feeding

I still need to return and write something about the food of these fish. They are predators, and therefore the main components of their diet are live fish . Any other food is just a substitute. Each species has, of course, slightly different needs. The greatest dietary specialist is Monocirrhus polyacanthus. The fish I kept accepted only live fish and live daphnia.

In very old literature, there are occasional mentions of substitutes. I have encountered this information in contemporary times as well. It is recommended to use strips of beef heart or fish fillet strips. One aquarist advised me to skewer these fillets and provoke the fish until they catch the meat. In the case of meat from warm-blooded animals, this is complete nonsense. Fish are not capable of perfectly digesting this food. The body temperature of cattle is 38 °C. Fish raised in water at a temperature of 24 °C - 28 °C accumulate undigested fats in their digestive tract, which usually leads to inflammatory diseases. In the past, I also fed some fish similarly during the winter months. Since I stopped this nonsense, I have healthy fish. A little better is fish meat.

Care of the eggs and methodological note

I write it as all aquarists know: the body of live fish consists of muscle, scales, skin, fins, bones, and digestive tract, where there is usually undigested, digested, and still unexcreted food. There are body organs such as the heart, kidneys, liver, swim bladder, eye, and also very important body fluids. I write this in such detail so that every aquarist realizes how important nutrients the predator deprives itself of. The result is after some time malnutrition and the resulting weakening of the organism and disease, which fish usually die from.

The administration of drastic medications usually only ends the suffering of the fish. Therefore, these fish should only be kept by aquarists prepared for the fact that over time they will also feed what they would otherwise sell. I want to conclude by wishing all those interested in keeping these fish success and joy that you will not experience with other fish.    

Text: Slavomil Boudný
Photo: Dr. Vladko Bydžovský