Focus on the commonly held Corydoras (2023)

Forever spoiled for choice, you can't go wrong with a Corydoras, says Julian Dignal.

There are many reasons why Corydoras are popular and have been an enduring part of our hobby almost since the beginning. There are hundreds of species to choose from, most are easy to keep, many are cheap, every fish shop has a few, and they even have a scientific name that's easy to pronounce - although it's usually insulted with kory!

Chances are if you have a community aquarium of almost any size, and even much more specialized settings like river habitats, there is a type of Corydoras for you.

So how do you choose this collection of common strains that often come up for sale?

I took a somewhat scientific approach and searched the 12,000 species of all catfish listed by their owners on my site to find the most commonly kept species.

"Bread and butter" fish is the term the trade gives to those species stocked by anyone with a dozen or so tanks. Among the goldfish, guppies, tiger barbels and neon tetras you will find the bronze cory (Corydoras aeneus). You will also find the Albino Cory, which is the same species, but its creamy white flanks and pink-red eyes appeal to many while not being attractive to others.

These fish are almost always farmed commercially, despite the fact that they - or a group of similar species - are widespread throughout tropical South America.

These are hardy fish and a great cheap choice for your first kory. Like almost all Corydoras, they should be kept in groups of at least three, with more being better. In fact, now is a good time to introduce some other Corydoras rules of thumb….

Easy gender determination

Adult males are always smaller than females of the species, but you will more often find them for sale, especially captive-bred populations, at a size that makes sexing easy. Males also tend to have more pointed fins, except perhaps the caudal fin.

Only a small percentage of species will have a different pattern or color between the sexes. Keeping two or three males per female is about right if you're planning a breeding project, but with all but the most mischievous species of Corydoras, this ratio isn't really important.

Different species will swim together - and even more so with Corydoras of the same shape or pattern.

They prefer to eat from the bottom and I'm sure part of their success in the hobby is because they like to eat flakes. You just have to make sure some have hit the bottom before them, or simplify it by adding a catfish tablet dipped in the Corydoras trio to your daily schedule. Scale up if you have more or bigger worms and feed flake food at the same time to make sure all fish get something.

Return to our bronze kory and under certain light these fish can really catch the eye as the metallic sheen of properly prepared fish gives rise to their common name.

If you see a "variety" of Corydoras with a bright red stripe on the lower half of the back, avoid it. These fish have been injected with dye and regardless of any argument that it affects vitality or longevity, it just doesn't feel right to me. You don't have to declare it to the shop - although I've seen it happen - just don't buy the fish!

I think this view kind of caught on, since I haven't seen a painted kori in a few years.

I once saw a large aquarium, about 6-6 feet long and 24 inches square on a side, fully decorated with wood and plants, but used essentially by the aquarist as a breeding tank for the wonderful group of copper bugs that regularly attend spawning.

School masses

In this tank there must have been at least a hundred, if not more, half-grown beetles - all offspring. This stuck in my head as they progressed like a school or two. They all moved at the same time or came to rest en masse. It was really quite a sight and probably the closest I've seen how these fish behave in large groups in the wild. I've been watching them for ages.

Anyway, it's long been stored away in that ever-expanding part of my mind where I keep ideas for new tanks...

I found it surprising that, at least among catfish breeders, Corydoras sterbai (above) is the second most commonly kept species. When this species was first introduced to the hobby it sold for high prices and even in the early 1990s breeding pairs could fetch around £50.

Fortunately, this species has proven easy to breed and the vast majority of individuals for sale have been bred in captivity rather than imported from Mato Grosso in Brazil. This is one of the bulkier strains that, in my opinion, has a pleasant, almost comical attitude.

Its popularity is also partly due to the attractive appearance of a creamy-white head and bright orange pectoral fin rays.

Although a good recommendation for the community aquarium, this species also thrives in warmer temperatures, thriving up to about 30°C/86°F. This means it is also a good choice for the well filtered, low water flow and high temperature environment of the average tray tank.

Discus also seem to appreciate this tank mate as they are not competitive for food and do not disturb the cute cichlids at night.

An albino form has recently become available for sale, and there is a very similar, though rarely imported, variety from Bolivia.

Again a widely available species, the Peppered cory, (C. paleatus) is not one to enjoy the warmer side of life in the aquarium. Decades of captive breeding have removed this trait, but stories circulate in catfish circles that this species occurs in the wild in streams where light ice has formed in the water.

I'll take this with a pinch of aquarium salt, but this species, introduced with many of the first imports to South America - and originally from the Buenos Aires region of Argentina - is actually from cooler waters.

Pepper was first discovered by a young Charles Darwin during his visit to Buenos Aires during his epic Beagle expedition in the mid-1830s. The species was later described to science by Jenyns in 1842 from the same expedition.

Wild specimens of this species are uncommon to find for sale and command much higher prices than the ubiquitous captive specimens found in almost all stores. The latter are very hardy fish, ideal for the regular community aquarium and often the first species of Corydoras, even catfish, that many fish keepers keep and even breed.

Fourth on my list is the Peruvian panda Corydoras - an easy scientific name to say and an easy species to keep. If I said this was a light brown and black fish, you might not think it was a particularly pretty species, but the panda-like black eye patch, black dorsal and a matching large black tail drop make it the most of a limited color palette.

Corydoras panda is a smaller species, the largest females do not exceed 5 cm, so it is easy to recommend a larger school of five or more individuals for your aquarium.

Like Corydoras sterbai, this strain created something of a storm when it first appeared, but soon proved easy to maintain and grow again. As a smaller species, this is an ideal companion for smaller tetras, dwarf cichlids and the like. In my experience, this species is one of those Corydoras that appreciates the inclusion of broadleaf plants for both shelter and resting.

Our hobby is full of cases of mistaken identity. It is a fact of taxonomic life that all organisms are reclassified, renamed, combined with, or separated from other species as more research is conducted and new species and regions of the world are discovered by scientists.

Before the internet, the names we used for our species came mostly from aquarium literature. However, they were often slow to change or adapt to the constraints of the scientific world, and names used decades ago—some of which were incorrect from day one—simply stuck.

Corydoras trilineatus has to be one of the most misunderstood species commonly sold. It is almost always sold as Corydoras julii or under the common name Julii cory. This is actually a global blunder.

Inherent errors

On a trip to the Amazon a few years ago, I set out specifically to find a group of very local Corydoras collectors who supplied the aquarium trade. When they sat down with these guys for the tour, they very unprompted gleefully used the terms "agassiz", "julee" and "elegans" to describe the awaited "corydora".

After a short boat ride and half an hour of stumbling through the rainforest, we had a successful but excruciating collecting session in which we caught several Corydoras - but for every one we caught, I'd swear we got bitten by 100 mosquitoes.

After hastily retreating to the huts which served as our base, we took a good look at what we had caught. 'Agassiz' turned out to be anything with large black spots and this trip included Corydoras ambiacus and C. leucomelas, and possibly juvenile C. agassizii.

We did not find any Corydoras elegans on this trip. Our 'julee' Corydoras were of course Corydoras trilineatus. It was interesting to see these species all living together - they are all caught in a single swing of a net.

The point of this story is that even collectors, deep in the Amazon, had picked up and kept this incorrect name from trade books and older aquariums.

Corydoras julii itself is a Brazilian species known from a much more restricted area and is a very unusual introduction. This is complicated by the fact that Corydoras trilineatus, which closely resembles C. julii, occasionally occurs.

Anyway, it's a name. If you find these fish for sale, they are almost certainly Corydoras trilineatus and hardy, mostly wild caught fish, again good for the community.

While all fish should be quarantined either before being sold or before being placed in your prize tank, most are not. Even if they are, they are often offered for sale in less than optimal condition.

Consider feeding new fish frozen bloodworms if you had them in the first few weeks. It will not spoil them and then make them unfavorable for cheaper food.

As a general rule, avoid Corydoras with bellies that are tight or that, unobstructed, do not rest on the aquarium floor.

The former means you're looking at malnourished fish, and the latter can have a number of causes - from uncertain new introductions to an indication of a water quality maintenance problem to an equally wide range of internal problems from parasites to organ damage.

Don't confuse this with something that all Corydoras do naturally, which is to rise quickly, usually vertically to the surface of the water, inhale a gulp of air and expel it as they settle.

At worst, it means there's a lack of dissolved oxygen in their environment, perhaps because it's a bit warm - but if that doesn't happen repeatedly, it could just be down to something they're just doing.

So-called nano-aquariums have recently carved out a niche area of ​​interest for our hobby, and Corydoras pygmaeus is one of the common dwarf or "micro" Corydora that are suitable to live in such an arrangement. Not surprisingly, it is commonly known as the Pygmy Cory and - along with other similar small species such as C. habrosus and C. hastatus - is something else again.

These species do not grow much larger than 3cm/1.2", again with males around 1cm/0.4" smaller.

In my experience plants are essential to these species and of all Corydoras these are most likely to be found in mid-water where in the wild they are often found among small silver carp with which they share more than a passing resemblance .

So I would recommend keeping them as they are and, because of their size, recommend a school of at least six, but preferably more. I have had excellent results keeping these pencilfish (Nannostomus spp.) in smaller tanks. I would attribute this success to the fact that they have a similar mouth size.

At this point, while they readily feed on sinking tablets and crushed flakes, don't be surprised if they encounter larger pieces - such is the sociability of the species.

C. schwartzi is another favorite of the aquarium hobby. I say consistently, but this is probably the top species among a group of similar species, including one or two that have yet to be described by science. This species is not commonly cultivated and most are imported. They are medium sized, with female dimensions approx 6cm/2.4".

So this is the common species, with a black vertical eye mask and horizontal rows of medium-sized black spots along the sides. These are compressed and may look more like lines than individual points.

With this type I find the quality of the imported files variable and sometimes you have to feed them on arrival. One problem I've had with Corydoras in general, and this species in particular, is that due to the double rows of armored scales covering their sides, they are quite immobile, at least compared to other catfish. The sturdier, as opposed to elongated, a particular species is, the more likely they are to stick to rock work.

Corydoras train fish and these schools have a startle reflex. During this risk dispersal, beetles can become fatally trapped on rocks. To a lesser extent, this also applies to the filter and heater components, although I haven't experienced this with them.

In general, I would suggest avoiding rock work with holes and hollows, as well as rock piles. If you really want to use rocks or stones in your layout, large round stones placed individually and slightly recessed into the substrate are best.

I have been running a school in Korydora brothers for almost 14 years and at least three are ten years old. In writing this article I discovered that I was not alone in doing this and that this is a species that is commonly kept and bred.

Unusual color

I like it because of the color and shape. It has the black eyestripe seen in many species, but also an unusually creamy body color and a noticeable orange patch on its head. It's like a variety of Tancho koi, but in a striped catfish, it's very easy on the eye. Corydoras adolfoi originates from the Rio Negro in Brazil and is a blackwater species, but again, especially captive bred animals have proven to be very adaptable.

I originally kept this species in a black water aquarium and they do very well at or around pH 5 with a sandy substrate, some driftwood and no plants. In nature, plants are not very common at this low pH and often the plant is only covered by vegetation that protrudes from the water rather than growing in it. I also found this species to be very active during the day.

If you are considering purchasing this species, try to find out if it is a wild-caught or captive-bred species. If the latter, you have less to consider when it comes to water quality, and I have yet to see a difference in color intensity of behavior when comparing wild-caught animals to several generations of captive-bred animals.

Also consider these...

There are other commonly kept species of Corydoras worth mentioning for those thinking of putting some in their tank. Among the commonly kept species are Corydoras arcuatus, pictured above. This is also a decent sized Cory, the curved black band from which it gets its name and is a firm favorite with many experienced keepers.

More recently, the introduction of Corydoras similis has led to it being bred in captivity and often available and kept. This crude species has several small black spots on its head, an attractive metallic sheen on its belly, and spots at the base of its tail.

If you have a little patience, Corydoras elegans is one of those species where males and females have different body patterns. In stores you usually only come across dull gray youngsters, but a well-groomed adult male must be one of the most beautiful sights in the world of Corydoras.

Again, buy a group of juveniles and enjoy the girls separating from the boys and the boys developing their colors.

One for the future?

Over the decades we have featured new or newly available Corydoras at great prices time and time again. These are eagerly adopted by breeders and often result in this strain becoming more and more available and affordable to the rest of us.

This is the case with the beautiful Corydoras weitzmani and if history repeats itself we will hopefully see this species become more available and fall well below the decade mark.

Let's hope so, but remember that this is all due to the efforts of the breeders and you should support them and their efforts to buy, keep and maybe achieve some breeding success yourself.

The article first appeared in the December 2008 issue of Practical Fishkeeping magazine. Reproduction is prohibited without written permission.