Focus on commonly kept Corydoras


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

There are many reasons why Corydoras are popular and have been an enduring part of our hobby almost since its inception. There are hundreds of species to choose from, most are easy to keep, many are inexpensive, every fish shop has some, and they even have a scientific name that’s easy to pronounce — although it is commonly contracted to cory!

Chances are that if you have pretty much any size of community tank, and indeed many more specialised set-ups like river biotopes, then there is a species of Corydoras for you.

So, how to shortlist a selection for this overview of the common species often offered for sale?

I’ve taken something of a scientific approach and sorted through the 12,000 species of all catfish registered by their keepers at my website to find those most commonly kept.

'Bread and butter' fish is the term given by the trade to those species that any shop with even just a dozen tanks will stock. Among the goldfish, guppies, Tiger barbs 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 pinkish red eye appeals to many as much as it is unappealing to others.

These fish are almost always commercially raised on fish farms, despite the fact that they — or a group of species similar — are widespread across all of tropical South America.

These then are hardy fish and an excellent inexpensive choice for your first cory. As with nearly all Corydoras they should be kept in groups of at least three, while more is better. In fact, now is a good time to introduce some other Corydoras rules of thumb….

Easy sexing

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

Only a small percentage of species will look any different in terms of pattern or colour between the sexes. Keeping two or three males per one female is about right if you’re planning a breeding project, but in all but the most boisterous of Corydoras species this ratio isn’t really important.

Different species will shoal together interchangeably — and with similarly shaped or patterned Corydoras even more so.

They prefer to eat from the bottom and I am certain part of their success in the hobby is because they will gregariously eat flake food. You just need to make sure some reaches the bottom for them, or simplify that by feeding your trio of Corydoras a sinking catfish tablet in your daily regime. Scale up if you have more or larger corys and feed at the same time as flake food to ensure all fish get something.

Back to our Bronze cory and in a certain light these fish can really catch the eye, as the metallic sheen of well conditioned fish give rise to their common name.

If you see a 'variety' of Corydoras with a bright red stripe in the lower back half, avoid them. These fish have been injected with dye and, irrespective of any argument around this affecting vitality or longevity, just doesn’t seem right to me. You don’t have to make a statement in the shop — although I’ve seen it done — just don’t buy the fish!

I think that view has rather caught on, as I have not seen a dyed cory for a few years now.

I once saw a large tank, around 152-183cm/5-6’ long and 61cm/2’ square at the side, fully decorated with wood and plants but it was, in essence, being used by the aquarist as a grow-out tank for his prodigious group of Bronze corys that were spawning regularly.

Shoaling masses

In this tank there must have been at least a hundred, if not more semi-adult corys — all of them offspring. This stuck in my mind as they moved as one or two shoals. All were moving at the same time or coming to rest en masse. It was really quite a sight and perhaps the closest I’ve come to seeing how these fish behave in large groups in the wild. I watched them for ages.

Anyway, it’s long been filed in that ever-expanding part of my mind where I keep ideas for new tanks…

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

Happily this species turned out to be easy to breed and the vast majority of individuals offered for sale are captive bred rather than imported from Brazil’s Mato Grosso. This is one of the more bulky species which, to my eyes, has a pleasant, almost comical demeanour.

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

While a good recommendation for the community tank, this species does well in higher temperatures too and will thrive up to around 30°C/86°F. That means it’s also a good choice for the well filtered, low water current, high temperature environment of your average Discus tank.

Discus too seem to appreciate this tank mate as it is neither competitive for food nor bothers the graceful cichlids during the night.

Recently an albino form has been available for sale and there is a very similar looking variety, though it is rarely imported, from Bolivia.

Again a commonly available species, the Peppered cory, (C. paleatus) is not one however that enjoys the warmer side of aquarium life. Decades of captive breeding have taken the edge off this trait but, in catfish circles, tales are told of this species being found in the wild in streams where a light ice has formed on the water.

I take that with a pinch of aquarium salt, but this species, introduced with many of the first South American imports — and originally from the area around Buenos Aires in Argentina — does in fact hail from cooler waters.

The Peppered cory was first 'discovered' by a young Charles Darwin when visiting Buenos Aires during his epic expedition aboard the Beagle in the mid 1830s. The species was later described to science by Jenyns in 1842 from that same expedition.

Wild-caught specimens of this species are unusual to find for sale and command much higher prices than the omnipresent captive-reared individuals you find in virtually all shops. The latter are very hardy fish, ideal for the regular community tank and often the first species of Corydoras, and indeed catfish, that many fishkeepers keep and indeed spawn.

Fourth on my list is the Peruvian Corydoras panda — a scientific name that is simple to say and a species simple to keep. If I said this is a fish with light brown and black coloration, you might not think it’s a particularly pretty species, but the panda-like black blotch on the eye, black dorsal and a matching large black dollop of a tail spot do make the most of a limited colour palette.

Corydoras panda is a smaller species, the larger females not getting past 5cm/2” and as such it is easy to recommend a larger shoal of five or more individuals for your tank.

Like Corydoras sterbai, this species created something of a storm when first introduced, but again was soon found to be easy to keep and breed. 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 broad-leaved plants for both cover and for perching on at rest.

Our hobby is littered with cases of mistaken identify. It’s a fact of taxonomic life that all organisms are reclassified, renamed, combined with other species or split out from them as more research is carried out and new species and areas of the world are uncovered by scientists.

Prior to the Internet, the names we used for our species were primarily taken from aquarium literature. However, these were often slow to change or adapt to the cut and thrust of the scientific world and names used decades ago — some incorrect from day one — have just plain stuck.

Corydoras trilineatus has to be one of, if not the most misidentified species commonly sold. It’s almost always sold as Corydoras julii or by the common name Julii cory. This is, in fact, a worldwide goof.

Native mistakes

On a trip to the Amazon a couple of years back, I specifically set out to find a group of very local Corydoras collectors that supplied the aquarium trade. Sitting down with these guys pre excursion, quite unprompted they happily used the terms 'agassiz', 'julee' and 'elegans' to describe the 'corydora' we could expect to encounter.

After a short boat trip and half an hour stumbling through rainforest we had a successful but excruciating collecting session in which we caught many Corydoras — but for every one we caught I’d swear we were bitten by 100 mosquitos.

After beating a hasty retreat to the huts that served as our base, we had a good look at what we had caught. 'Agassiz' turned out to be pretty much anything with big black spots and on this trip included Corydoras ambiacus and C. leucomelas, and perhaps also young C. agassizii.

We found no Corydoras elegans on this trip. Our 'julee' Corydoras were of course Corydoras trilineatus. It was interesting to note these species all live together — all being caught in the single swing of a net.

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

Corydoras julii itself is a Brazilian species known from a much more restricted range and is a very uncommon import. This is complicated by the fact that from time to time Corydoras trilineatus, that look very similar to C. julii, do turn up.

Anyway, it’s a name. If you find these fish for sale, they’ll almost certainly be Corydoras trilineatus and hardy, usually wild-caught fish, again good for the community.

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

Consider feeding new fish up on frozen bloodworm once you have had them for the first few weeks. It will not spoil them and make them subsequently adverse to cheaper foods.

As another rule, avoid  Corydoras with pinched bellies or those that, unmolested, do not  rest on the aquarium floor.

The former condition means you are looking at malnourished fish and the latter can have a number of causes — ranging from unsettled new imports through to indicating a problem with holding water quality, to an equally wide range of internal problems from parasites to organ damage.

Don’t confuse this with something that all Corydoras do naturally, and that is rapidly rise, usually vertically to the water surface, take in a gulp of air and expel it while coming back to rest.

At worst this means there is a scarcity of dissolved oxygen in their surroundings, perhaps because it’s a bit warm — but if not done repeatedly, this can be put down to simply something that they just do.

So-called nano aquariums have recently carved themselves a niche interest area in our hobby and Corydoras pygmaeus is one of the common dwarf or ‘micro’ Corydoras very much suited to life in such a set-up. Unsurprisingly, it’s commonly known as the Pygmy cory and it  — along with other similar small species such as C. habrosus and C. hastatus — are something a bit different again.

These species don’t get much bigger than 3cm/1.2”, again with males about 1cm/0.4” smaller.

In my experience, plants are a must for these species and out of all Corydoras these are the most likely to be found in midwater where, in nature, they can often be found among small silver characins with which they share more than a passing resemblance. 

So I’d recommend keeping them as such and, because of their size, propose a shoal of at least six, but better more. I’ve had great results keeping these fish with pencilfish (Nannostomus spp.) in smaller aquaria. I’d put this success down to the fact they have similarly-sized mouths.

On that subject, while easy to feed on sinking tablets and crushed flake, you shouldn’t be surprised if they tackle larger morsels — such is the gregariousness of the species.

C. schwartzi is another stalwart of the aquarium hobby. I say stalwart, but this is perhaps the flagship species among a group of similar ones that may include one or two not yet described to science. This species isn’t commonly bred and most are imported. It’s of  medium size, with females around 6cm/2.4”.

This then is the common species, with a black vertical eye mask and horizontal rows of medium-sized black blotches along its flanks. These are compressed together and can look more like lines than individual spots.

With this species I find the quality of imported stock variable and sometimes you need to feed them up on arrival. One problem I’ve had with Corydoras generally, and this species particularly, is that due to twin rows of armoured scales that cover their flanks, they are, relative to other catfish at least,  quite immobile. The more robust, as opposed to elongate, a particular species is then the more likely they are to get trapped by rockwork.

Corydoras are shoaling fish and these shoals have a fright reflex. During this scattering from danger corys can get fatally stuck in rocks. To a lesser degree this is also true with filter and heater attachments, although I’ve not experienced this happening with them.

Generally, I’d suggest avoiding rockwork with holes and cavities, as well as piles of rocks. If you really want to use rocks or stones in your layout, big round rocks individually placed and slightly sunk into the substrate are best.

I’ve been keeping a shoal of Corydoras adolfoi for nearly 14 years and at least three are ten years old. In writing this article I found that I was not alone in doing so and this is a commonly kept and bred species.

Unusual colour

I like it because of its coloration and shape. It has the black eye stripe we see in many species, but also an unusually creamy body coloration and a remarkable orange spot on its head. It’s like a variety of Tancho koi but in a catfish with stripes is very easy on the eye. Corydoras adolfoi comes from Brazil’s Rio Negro and is a black water species, but again especially captive-raised stock have proved very adaptable.

Originally I kept this species in a blackwater tank and they really do well in or around pH 5 with a sandy substrate, a little bogwood and no plants. In nature, plants don’t feature much in that low a pH and often plant cover is only provided by vegetation overhanging the water rather than growing in it. I also found this species to be very active during the day.

If considering buying this species, try to find out if it is wild-caught or captive-bred. If the latter you will have less to consider in terms of water quality and I’ve yet to see any difference in intensity of colour of behaviour in comparing wild-caught versus several generations of captive-bred stock.

Consider these too…

There are other commonly kept Corydoras species  worth a mention for those thinking about putting a few in their tank. Among those commonly kept are the Arched corydoras, (Corydoras arcuatus), pictured above, which is another fair-sized cory whose arched black band gives it its name and is a real favourite with many experienced keepers.

More recently the introduction of Corydoras similis has seen it become captive bred and frequently available and kept. This sandy species has many small black spots on its head, an attractive metallic sheen on its back body and  smudge spot at the base of its tail.

If you’ve a little patience, Corydoras elegans is one of those species where males and females have different body patterns. You tend to only encounter drab grey youngsters in the shops, but a well- conditioned mature male has to be among one of the most splendid sights in the Corydoras world.

Again, buy a group of juveniles and have fun watching the girls separate from the boys and the latter developing their colours.

One for the future?

Time and again over the decades we’ve see new or newly available Corydoras introduced at high prices. These are gamely taken on by breeders and often result in that species becoming increasingly available and affordable to the rest of us.

This is happening with the beautiful Corydoras weitzmani and, if history repeats, we will hopefully see this species become more available and drop well under the tenner a fish mark.

Let’s hope so, but don’t forget this is down to the efforts of breeders and you should support them and their efforts in buying, keeping and perhaps having a few breeding successes of your own.

The article first appeared in the December 2008 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.