Dwarf Corys

Dwarf corys are the ideal choice for the classic 24-inch (60cm), 15-gallon (54-L) tank. They are peaceful, active during the daytime, and easy to maintain. They can be reproduced quite easily because they do not eat their offspring.

In short, if you like nano-scale fishes and only have a bit of space you should try dwarf corys. This article introduces the most common species.

Among the more than 400 catfish species of the subfamily Corydoradinae (family Callichthyidae) that colonize the numerous South American river systems, we know a variety of shapes and sizes. They range from the large, bulky species of the genus Brochis, some of which can grow up to 6 inches (15 cm) or more in total length, down to absolute dwarfs just 0.8-1 inch (2-3 cm) long. The latter are the subject of our attention here.

I will look primarily at the classic four species and mention other candidates only in passing. The size range of the Corydoradinae is continuous, and only Aspidoras pauciradiatus, Corydoras habrosus, C. hastatus, and C. pygmaeus are true dwarfs, even when compared to some of their relatives, who are not exactly huge either. There are a number of other tiny corys, but aquaristically they play minor roles. Species such as C. multimaculatus or various Aspidoras species only reach a few centimeters in length as well, but the females of these species become significantly larger than the females of the aforementioned dwarf species. Aspidoras depinnai, A. sp. C35, and A. microgalaeus have been maintained now and then, but they have almost disappeared from the hobby. Their habitats are outside the traditional collecting grounds of the exporters. I collected these and other Aspidoras species in the 1990s and introduced them into the hobby. However, the breeding talents and good will of a few committed aquarists proved insufficient to maintain these species for decades in the hobby. Another group of dwarf corys, three species from the Brazilian shield, are presented in the next article.

Free-swimming corys

I take for granted that each species will be housed in a single-species aquarium. This is the best way to obtain fry without much intervention. Of course, all dwarf corys can be kept in community tanks with other dwarf fishes. Dwarf tetras and danios are often combined with them, so that something is always swimming in the open water. When they feel comfortable and safe, A. pauciradiatus, C. hastatus, and even C. pygmaeus move into the middle water level, where they are easily observed as a group. However, if there is too much traffic above them, they will be timid about coming out into open water.

Other corys, such as C. guapore and related species, also commonly swim in the middle water layers, while most species only do that when they are in spawning mode. Even between the four species presented here, the behavior is quite different. Especially during the spawning period, the dwarfs can be constantly observed in the open water.

I have already noted that the four species featured here are easy to maintain and breed (with one exception), but certain basic conditions must be provided when setting up the aquarium.

All four species can be easily accommodated in a group of 10 to 20 specimens in an aquarium 24 inches (60 cm) long. As with almost all corys, the substrate should consist of fine sand. The layer does not have to be thick; a couple of centimeters (an inch or less) will suffice. If you want to grow heavily rooting plants and agree with the general consensus, a fine and deep substrate is not beneficial. I have found that Cryptocoryne, Anubias, Java Fern, and various stem plants also do well without any substrate. The roots then form a dense network, where the newly hatched juveniles hide and find tiny bits of food. I like to fill the background and edges of the aquarium with some of these plants and put Java Moss, Vesicularia dubyana, or other undemanding mosses on the sand layer in the center of the tank. The moss spreads fast and often forms a dense carpet that the dwarf corysappreciate. I leave the middle area uncluttered so the group can be easily observed there. For C. habrosus a larger open area is needed, or the aquarist won’t even get to see these dwarf corys.

I like to add a few floating plants to shade the aquarium and give the animals a feeling of security. I have successfully kept C. habrosus with C. hastatus in the same tank, as both species avoid each other and the groups do not mix. In such an aquarium, I got fry from both species.

The combination of C. habrosus with C. pygmaeus also worked, but the groups mixed and the C. pygmaeus did not swim freely. So far, I have not bothered to combine the three free-swimming species because I did not want the swarms mingling. Aspidoras pauciradiatus are also rather territorial among themselves during the spawning season. However, these animals are hardly competitive with other species, which is why I always keep them in a species aquarium.

Spawning after a water change

Depending on the population density in the aquarium, every one to two weeks I change 30 percent of the water with temperate tap water. I have reproduced all of the species mentioned here, including the black-water species A. pauciradiatus, in medium-hard water with a pH of 7-8. At least in the case of A. pauciradiatus, the animals welcome alder cones and beech or other tree leaves that release tannins in the water and stain it brown.

I mainly feed small foods. Conventional dry foods are gladly accepted. In addition, I offer live Artemia nauplii, frozen Cyclops, and small Daphnia. I no longer feed live Cyclops, since some attached themselves to the catfish—and they did not appreciate this.

The optimum water temperature differs depending on the species and its origin. Corydoras hastatus is very widespread and has settled large areas of tropical and subtropical South America south of the Amazon. The animals do well even at low temperatures around 72°F (22°C), while C. pygmaeus and A. pauciradiatus should be kept warmer. Corydoras habrosus is the most heat-loving species of these four and spawns productively at 79°F (26°C) and above.

If you take a little time, you can watch your dwarf corys spawn, especially after you perform water changes. Initially, C. habrosus may need a longer period without a water change and then a slight drop in temperature to be stimulated. However, once the group gets going, the spectacle will be repeated after the water change every few weeks.

I will describe the spawning of the four species separately, since there are significant differences. But they all have one thing in common: juveniles appear without major effort by the aquarist if Artemia nauplii or other tiny foods are fed regularly. Often you see the little ones only when they have been integrated into the parental group. Quite a few aquarists are amazed at how quickly a group grows to more than 50 animals.

The babies offer a great show for visitors. If you lift the clump of Java Moss, many tiny striped baby corys dash in all directions. However, an excess of snails or a Planaria infestation ends this spectacle quickly. In that case, treating the aquarium with flubendazole will dispose of these pests safely without harming the fish larvae. (This anti-parasitic drug can persist in a tank, however, making it impossible to keep more desirable snails.)

All dwarf corys mentioned here can be purchased as wild imports from the trade, depending on the fishing season in the country of origin. With some luck, you might even find a breeder to sell you offspring.

Corydoras hastatus

Older aquarists may have known the late German catfish expert Hanns-Joachim Franke. The dwarf Corydoras, dwarf catfish, Tail-Spot Pygmy Catfish, or micro catfish (C. hastatus) was high on Hanns-Joachim’s wish list. He often asked me for these dwarfs, and he was very happy when I was finally able to get him some animals.

Up until the 1990s, this species was barely available and was constantly confused with C. pygmaeus. Even today, fish importers ordering C. hastatus not infrequently receive the other species, even though these species can be clearly distinguished from others. I will not provide a detailed description, because the images speak for themselves.

The story of the first description of this small catfish is rather murky. Eigenmann & Eigenmann (1888) described the species based on several specimens that the Swiss-American naturalist and ichthyologist Louis Agassiz had collected on his trip to Brazil in the years 1865-1866. The type locality, Villa Bella, is today the ever-expanding city of Parintins, on the southern bank of the Amazon in the state of Para.

I have taken the trouble to read what Agassiz wrote in his travel diary about Villa Bella. He describes very clearly how the gentlemen explorers of the time roamed the region with boats and canoes and encouraged the resident Indians to collect fishes for them. The sampled fishes are not described in detail. I have fished around Parintins myself, but I could not catch a single C. hastatus there. However, much further south, in the drainage of the mighty Rio Paraguay, the species is often found in swampy areas and calm parts of larger rivers. Knaack (2000) mentioned localities in the northern and southern Pantanal of Brazil and Bolivia, in Paraguay, and in northern Argentina. I collected the species in the Pantanal and in the northern Argentine province of Misiones, where it is very common in the drainage of the Rio Parana as well.

Eigenmann and Ward (1907) described a dwarf catfish from the Rio Paraguay in Corumba as Corydoras australe, which was considered a synonym of C. hastatus by Gosline (1940). Nijssen and Isbrucker (1980) then defined a lectotype for C. hastatus in their revision of the genus. Azpelicueta and Yanowsky (1992) finally examined specimens from Argentina (Rio Paraguay, El-Bagual Nature Reserve) and revalidated C. australe due to the different numbers of soft rays in the dorsal fin. Thus, C. hastatus has six soft rays and C. australe has seven. This feature alone is hardly a reason to accept both species as valid, so C. australe (Eigenmann & Ward, 1907) is commonly considered a synonym of C. hastatus.

Incidentally, the lectotype designated by Nijssen and Isbrucker and collected by Agassiz in Parintins has seven soft rays. In the trade, mainly the form with seven soft rays appears (take a count on available photos!). Only on some animals imported from Brazil do we occasionally find six soft rays.

Imaginative aquarists also see differences in coloring or patterning among their animals—sometimes the stripe on the lower abdomen is darker and sometimes it is not.

The animals look much nicer when spawning, when the patterns, including the rarely shown black abdominal line, appear much more vividly against the metallic golden background. All this may be cause for speculation about potentially distinct species, but without documented studies of individual local variants this discussion leads nowhere.

Enough about fin rays and theories. Aquarists are interested primarily in the practical aspects. These include observations in the natural habitat, and in that respect, C. hastatus has a lot to offer.

Madam General

During the high-water season, C. hastatus is found in small groups. Previous reports and my own experiences have found that this catfish lives mainly in the shade of floating meadows, with their dense growth of bouyant plants and their roots.

The high-water season is also the time of reproduction. Small groups of 10-15 animals swim together in the clear water in a small area, the center of which is usually formed by a large Eichhornia or another dense cluster of floating plants. The animals “stand” together, forage the roots in search of food, and spawn regularly.

Such a group is dominated by an exceptionally large female (well over 1.5 inches/4 cm) and three to four spawning males. Knaack (2000) reported that the animals spawned at their “home plant” over a period of four to six weeks and were very faithful to that location. The large female, “Madam General,” is clearly the boss of the group, spawning constantly with different partners.

Knaack’s observations refer to animals both in the Pantanal and in the drainage of the Rio Guapore in Brazil.

Unfortunately, I did not have the opportunity to make such long-term observations, but I observed another phenomenon: at all times, even in the reproduction-free dry period, Corydoras hastatus forms large (sometimes huge) or small groups with the similarly patterned tetras of the genus Serrapinnus. Knaack also described this phenomenon.

While snorkeling in the relatively clear waters of the Rio Mutum in the Pantanal during the high-water season, every few meters I could see a small group of C. hastatus spawning under the floating meadows. If you collect such an established group and put it into an aquarium, spawning resumes within a few days. Arbitrary groups, such as those acquired from the pet trade, need longer to acclimatize and establish rank order. I could also never make out a “Madam General” among tank-raised groups in the aquarium. Is this behavior lost in captivity?

During the dry season, the water level drops significantly, and C. hastatus can be found in large numbers in the residual pools. Only then can the species be caught for export. Exports today come from Paraguay and, rarely, from Brazil.

Egg for egg

During spawning in the aquarium, the whole group of C. hastatus whirls in the open water. A short display by one or more males can be observed every day. Every now and then, a pair separates into the plants to spawn. After a short chase, the pair ends up in the Corydoras-typical T-position.

If you give the group a rest period for several weeks at 72-75°F (22-24°C) without a water change and then offer them a protein-rich diet and change 20-30 percent of the water every one to two days while raising the water temperature a bit, you can control the spawning very nicely.

Some breeders make the effort to collect the individually deposited eggs. This disturbs the breeding group unnecessarily and is also not useful. In most cases, the juveniles are more delicate and extremely skittish when raised separately. It is much better to leave the fry with their parents. This method is always preferable, especially if not that many fry are desired. Even then, enough fry make it, and soon the time comes when they need to be thinned out.

After a one- or two-month spawning period, I usually stop doing water changes, siphon out the numerous large juveniles, and raise them separately. It seems important to retain sufficient fry for the future. Many breeders complain regularly about the increasing frailty of their animals; it is not uncommon to lose an entire group after one or two years. This has happened to me occasionally, especially when I kept too many animals together in a small tank. In a 50-gallon (200-L) aquarium, I never observed that, even when there were more than 100 fish. Apparently, the population regulates itself if it is too dense. This appears to be genetically encoded; it can only be avoided if you offer the animals enough room.

Corydoras hastatus is certainly a very interesting fish to keep, and not just for catfish devotees. There are also other species that are worth a try.

Corydoras pygmaeus

Corydoras pygmaeus probably suffers the fate of forever coming second. Knaack (1966) described the species from the Rio Madeira in Brazil after he realized that these armored catfishes were significantly different in coloration and body measurements from the C. hastatus that he had kept. Correspondingly, the first description is quite short. C. hastatus is clearly the more popular of the two. To make matters worse, C. pygmaeus was also the second species of dwarf cory that I was able to get my hands on. But let’s start at the beginning.

In addition to the Rio Madeira, Peru has been named as the country of origin for these dwarfs. Nijssen and Isbrucker (1986) mentioned the upper and lower Rio Napo in Ecuador and Peru and some localities near Iquitos, among others the Rio Nanay. The Rio Nanay is a backwater river, while the other localities are whitewater streams.

Like C. hastatus, which is found in habitats of very different water chemistry and easily survives sudden water changes (such as heavy rainfall in relatively small water bodies), C. pygmaeus is not very demanding interms of water quality. Medium-hard tap water with a neutral pH, which is common in many places, is acceptable for these animals. The water temperature should be somewhat higher than for C. hastatus, as C. pygmaeus is a true tropical fish and thrives in water temperatures between 77 and 86°F (25-30°C).

For care and breeding, the same applies as for C. hastatus. However, the problem of crashing populations at high density is not an issue.

Apparently, C. pygmaeus is the less sensitive species of the two. Well-fed animals readily reproduce with increased water changes and a slight decrease in temperature, which together mimic the rainfalls during high-water season in the natural habitat.

The keeper quickly notices when C. pygmaeus spawn. The entire group whirls in the open water and trios or quartets, each including a female, dash through the tank. Eggs are released one at a time; with three or four spawning females in the group, about 100 eggs result every second or third day for weeks. An ambitious breeder can produce large quantities quickly if he uses a mop and replaces it regularly after spawning.

Unlike C. hastatus, the commercial breeding of which usually fails due to the low rate of fry survival, C. pygmaeus is relatively easy to reproduce in large quantities. The only question is who will want the many small catfish after three or four months, when they have reached sale size. Corydoras pygmaeus needs a month or two longer to mature, while C. hastatus begins spawning at the age of four or five months. A breeding tank that contains several hundred dwarf corys swimming in a loose flock in the open water is a sight to make any breeder proud.

For all the dwarf species listed here, we are inclined to anticipate only a short life expectancy. It is quite possible that these dwarfs are only annual in nature, but in the aquarium they live much longer. My oldest C. pygmaeus lived for five years. He was a bachelor who had lived for years among larger corys in a spare tank.

The undervalued Aspidoras pauciradiatus

Aspidoras pauciradiatus is a small, handsome catfish that actually does not really fit into the genus Aspidoras. It has been regularly imported in large numbers at certain times for many years. Like all dwarf armored catfishes, this species has a reduced number of pectoral and dorsal fin rays, which has led to its name (Latin paucus = few, radius = ray). At that time, it was still included in the genus

Corydoras (Weitzman & Nijssen, 1970). The type locality was given as the middle Rio Araguaia near Aruana in the Brazilian state of Goias.

The type material for the description was collected by Herbert R. Axelrod, who traveled to Brazil many times. Apparently, he made a mistake, because A. pauciradiatus definitely does not live in the middle Rio Araguaia. I have made two trips, in 1998 and 1999, to the area of Aruana, but found these animals nowhere and asked all the professional aquarium fish collectors in vain about the species. Actually, Aspidoras pauciradiatus comes from the middle Rio Negro near the small town of Barcelos. It has been collected there for generations, along with the Cardinal Tetra and other species, and exported via Manaus.

The species lives in shallow, sandy shore areas among leaf litter, where they search for food. Lorenz (2010) caught the species near Barcelos and was able to document that A. pauciradiatus is a blackwater species. The water had a pH of 4.5 and a conductivity of 16 S/cm. The water temperature was very high at 86°F (30°C), which is important to know. Aspidoras pauciradiatus is thermophilic and requires temperatures of 79-86°F (26-30°C) to do well in the aquarium.

The species spawns in tap water, but rearing the delicate fry succeeds much better in slightly acidic and quite soft water. Various hobbyists have reported that juveniles have suddenly appeared in their aquariums.Scottish aquarist Alan Pinkerton was the first to report a successful breeding of this species (1987). The tap water in Scotland is mostly very soft and is ideal for breeding South American fishes.

Spawning spectacle

My targeted experiments with A. pauciradiatus were done a few years back, and I recently dug up my old notes again. At that time (2004), I had set up a group of 20 animals in a 53-gallon (200-L) species aquarium, as described above. The water hardness was 4°dGH, the pH was 6.5, and the temperature was kept at 83°F (28.5°C) for 12 weeks. During this time, no water changes were done and they were fed sparingly.

The animals were relatively calm and foraged the substrate for food in small groups in the morning and evening hours. Artemia nauplii and food tablets were served. Beginning in mid-June 2004, I changed 20-30 percent of the water every two to three days, and the water temperature dropped to 79°F (26°C). This imitation of the high-water season with the onset of rains was effective. The females became noticeably gravid and, starting in the second week, the entire group swam together in the middle water layer. The external filter was cleaned and that increased the current in the aquarium. The second phase began and the whole group whirled excitedly through the open water.

More and more often,

I could now see the smaller males as they swam vigorously up and down the rear wall, while the females became perfectly round. Now I increasingly fed Grindal worms and live Daphnia. However, the males did not chase the females; they just frantically swam along the walls for 10 days.

Soon the behavior of the males changed and phase three began. The three strongest males occupied three well-demarcated territories along the edge planting. Inferior males were nudged and downright chased away by the territory owners. The owners liked to sit on a small rock or a root to have a better overview from their elevated platforms. The females swam first with the uninvolved males in the center of the tank, but this behavior also changed over time. The thick females often rested alone on the bottom or lazily swam toward the territorial males who were also resting on the bottom.

When a territorial male saw a female, he sprang into action. He swam toward the female and touched her head region with his barbels. That was probably a stimulus; in any case, the pair then swam together into his territory and later into the dense edge vegetation. Often it was the female who quickly darted into the vegetation upon repeated touching of her head region by the male.The male immediately followed. This all happened very fast, especially after the initial warm-up period, and in all three male-occupied territories simultaneously. The courtship behavior of C. pygmaeus and C. hastatus happens at a snail’s pace compared to that.

In the vegetation, presumably the T-position and spawning followed. In any case, the female came out of the plants with one or two eggs in her pelvic fin pouch. She put the eggs on the exit tube of the filter, and I collected them every night during the next few days. After depositing the eggs, the female swam back to the tank’s center and the game began again. Eventually, I lost track of individual fish in all this whirling about, but I think that the females preferred certain males or always chose the same male.

After about a week, all the females were thin again and they slowly calmed down. Several hundred eggs were the result, but my fry-rearing attempts were met with only limited success. The young fish are more sensitive than those of the other species, and they only eat Artemia nauplii after about a week. Before that, I added detritus, Paramecium, and a large bunch of Java Moss to the breeding tank. However, I could raise only about 30 fry. I must conclude that A. pauciradiatus is a challenge for the breeder!

Salt and Pepper Cory

The final dwarf armored catfish in this group is a species that I came to appreciate when I was a student. It used to improve my pocket-money situation, since it was a hardworking spawner. Once referred to as Corydoras cochui, it is now clearly identified as C. habrosus. This fish has the fitting German name “Schachbrettpanzerwels” (Checkerboard Cory), but is known in the English language as the Salt and Pepper Cory.

Stanley Weitzman (1960) described C. habrosus based on material from the drainage of the Rio Pajo Viejo in the state of Cojedes, in the heart of the Venezuelan llanos. Corydoras habrosus also lives in the Colombian portion of the llanos, a huge savannah that turns into a giant swamp during the rainy season and is drained by the mighty Rio Orinoco.

For decades, C. habrosus has been regularly imported from Colombia, and it is seasonally available in pet shops. The patterns of the animals are highly variable. Most wear a black stripe along the flanks of the body, but this can be interrupted, sometimes only on one side of the body. The patterning on the specimens with broken stripes looks like a checkerboard—hence their name.

A group of 10 animals, housed as described above, can be a lot of fun for any aquarist. Even beginners in the care and breeding of catfishes can try breeding them and earn their spurs. Among the dwarf corys, C. habrosus have the largest eggs. The fry are easily raised with freshly hatched Artemia nauplii after the absorption of the yolk sac. Again, only one egg is released per mating and the eggs are distributed throughout the tank.

In its natural habitat, C. habrosus is exposed at times to extreme environmental conditions. In strong sunlight, the residual water pools often heat up to 91°F (33°C) or higher. The animals survive this low-oxygen period only by respiring atmospheric oxygen. Many armored catfishes have the ability to absorb oxygen via the intestinal epithelium. They rise to the surface and swallow air. Fish-eating birds, of course, notice this, but they often choke on the fish’s sharp pectoral fin rays. The Salt and Pepper Cory is thus well adapted to a life in the South American plains.

When the heavy rains set in and the air and water cool down considerably, the breeding season for the fish of the llanos begins. Remember these clues and you will know how to “persuade” C. habrosus to spawn. I hope that you have fun with these little eye-rollers. Go ahead— bring on the rains and the breeding season!

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