Bottom of the Tank: Captivating Cories

Author: Joshua Wiegert

Catfishes of the genus Corydoras and its closely related genera are the catfishes of choice for many aquarists. One reason is that cory cats are an incredibly diverse group, one that includes species representing every level of fishkeeping, from relatively easy-to-keep species to incredibly delicate ones.

Let’s examine this catfish group, taking a closer look at a handful of species on the scale from easy to keep to delicate. Corydoras are often dismissed by novice aquarists as simple bottom feeders, and often only one or two of these interesting fish are put into a community aquarium. This is unfortunate, as cory cats not only school but also begin to show interesting behavior when kept in larger numbers. In their native waters, they often form massive schools of mixed species, with quite a bit of interaction occurring between both individuals and the various species in the school.

Cory Basics

Cory cats are capable of absorbing swallowed air through the walls of their intestines. They will often rush toward the surface of the water and take a quick gulp of air. In some cases, this is obligatory; in other words, cory cats can drown if not allowed access to the surface. Therefore, one should consider the maximum water depth and also the availability of the surface when placing cory cats into an aquarium. If the tank is so deep that they cannot reach the surface, or if there is very little surface to reach, the fish can be in trouble. I had a customer who routinely killed cory cats, and we eventually determined that it was because the fish were being kept in an acrylic aquarium with an essentially solid top and almost no access to air. Even though the water was well oxygenated, the cories could not survive.

Pectoral Fins

The sharp pectoral fins of Corydoras bear mentioning as well. When netting these fish, always take care to avoid getting these fins caught in a net. More important, take care to avoid getting them stuck in your hand! Not only are the fins sharp, but most cories possess a mild venom that can cause pain for several hours. It isn’t likely to land you in the hospital or anything of the sort (unless you have an allergy), but it hurts like a bee sting.

Skin Toxins

Although there is some debate as to it in scientific literature, those of us who work with live cories—especially those of us who ship them—have no doubt that many species of Corydoras are also able to produce a skin toxin when stressed. This skin toxin undoubtedly serves a predator-deterring effect in nature, but in the confines of an aquarium (or a shipping bag), stressed cory cats can release a lethal dose of toxins. Some species appear to release little or no toxins, while others are infamous for their ability to kill themselves in shipment. For this reason, it is very important, when purchasing cory cats, not to add their bag water to your aquarium. (I believe that you should never add the water from the bag acquired with any new fish to your tank, but in this case, it becomes even more important.)

More Than Cleaners

Too often, the cories are viewed as simple “cleaner fish,” with the idea that they can survive off the leftovers from feeding community fishes. This is problematic for two reasons. First, if there is “leftover” food in the aquarium, the aquarist is overfeeding the tank in the first place, and adding something that will supposedly clean it is not the solution; the solution is to feed less. There is no substitute for proper feeding and aquarium husbandry—clean the tank!

Second, these interesting catfishes should not be treated just as the aquarium janitors. They will do best when fed a sinking food, particularly in the evening. There are countless sinking pellets, wafers, and the like on the market. Many quality frozen foods also sink and can provide an excellent diet, especially if you are attempting to breed these fishes. There is no substitute for clean, living blackworms for cory cats, either (and the other fish can clean up what the cories miss!).

It is important that cory cats be maintained in a clean aquarium that regularly receives gravel vacuuming. Built-up organic nutrients, particularly at the tank bottom, can cause health problems for these fish. Gravel should be fine and smooth, as sharp stones can cut a cory’s snout as it probes the gravel for food.

Species of Cories

The genus Corydoras contains some 160-plus described species, with several associated species now in other genera (e.g., Aspidoras, Scleromystax). In addition to these described species, there are countless other undescribed species that are often traded in the hobby under either a C-number or a CW-number. Many of the described species are also generally thought to represent multiple species, with potential new species being created from these complex species. This is an incredibly diverse group!

Corydoras Aeneus (Gill 1858)

Undoubtedly the best-known Corydoras species is Corydoras aeneus, the bronze cory. This is the fish that is most often labeled as “cory cat,” or something similar, in aquarium stores. While most of our stock is now farmed, wild bronze cories are quite beautiful and variable.

C. aeneus likely represents a complex of several different races, with some of the variants having been described at one point or another as other species. Many aquarists feel that these forms should be re-elevated to species rank.

The bronze cory has a light greenish-orange body (hence the name) with dark stripes along the flanks. An albino version is widely traded, and most of the “albino corydoras” we see in the trade are likely C. aeneus. Several other Corydoras also have albino forms, and these forms can be truly tough to tell apart, though some have telltale signs.

The bronze cory reaches a maximum size in the aquarium of about 3 inches (7.5 cm) or so and is generally quite forgiving of aquarium conditions, particularly when dealing with farmed fishes. These fish will really show their best in a small group and should not be maintained in numbers fewer than six fish.

They are remarkably easy to breed, with males appearing slimmer (when viewed from the top) than the relatively fat females. It is not unusual for these fish to spawn in the aquarium without any interference from the aquarist, but they can easily be induced to spawn. Feed a diet heavy in proteins, such as living blackworms, and watch as the females fatten. Ideally, males should outnumber the females.

Spawning of the bronze cory is triggered by storms, which seem to be a very common trigger among cory cats in general. A storm can be simulated by a large water change with cool water. If this is done at about the same time as a natural weather event, which will have a corresponding change in atmospheric pressure, the fish will virtually always spawn. The water entering the tank should be about 5 to 10°F (9 to 18ºC) cooler than the aquarium temperature, as you don’t want to shock the fish but just cool them down a little bit. Water change amounts ranging from 50 to 75 percent are ideal.

Males will follow a female around while courting her. They present their abdomen toward her head, creating what is known as the “Corydoras T-position.”

It has been shown in laboratory experiments that the male’s gametes travel through the female’s intestinal tract, exiting with the eggs as she deposits them. In the wild, eggs are laid on plant leaves, but in the aquarium, smooth surfaces tend to be preferred, especially the aquarium glass. The eggs are white. They’re remarkably sturdy and can be removed to a fry-rearing tank by being scooped off the glass with a razor blade. The eggs can also be left on the wall in the tank with the parents. If the parents are reasonably well fed, they will eat some, but not all, of the eggs, and the fry can develop in the breeding tank with the adults. If you really want to successfully raise these fish, moving the eggs to a fry-rearing tank is a good idea.

A 10-gallon (38-liter) aquarium with a seasoned sponge filter will often suffice. The water should just barely cover the surface of the sponge filter (remember that they need to get air, and fry are small). The tank can have a bare bottom or a fine layer of sand. The fry will grow rapidly when fed microworms, small pellets, and so forth. I generally do water changes weekly and add about a gallon of water extra each time until the tank is filled. At that point, the fry can be transitioned to a larger tank.

C. Sterbai (Knaack, 1962)

One of the more colorful varieties of Corydoras catfish is the orange-fin cory, which has become fairly available and reasonably priced, making it our medium-range cory. This Corydoras has an interesting honeycomb pattern of black and gold spots, with the gold being somewhat variable toward white in some individuals. Most striking of all, the pectoral and pelvic fins can range from slightly orange to bright orange (thus the common name). Oddly enough, an albino version of this fish has been developed; it retains some orange in the fins.

C. sterbai can reach a maximum size of about 2½ inches (6 cm). Like the bronze cory, C. sterbai is predominantly produced commercially nowadays, with very few fish actually being exported from the wild. These fish school together fairly well, and they also seem to leave the bottom frequently. They can be kept in a well-planted tank, where they’ll often perch on plant leaves, driftwood, and other decoration.

The orange-fin cory is quite similar in husbandry to C. aeneus, though it can be a little less forgiving about poor water conditions. It can be bred in much the same fashion, though eggs tend to be laid in areas of higher water flow. Orange-fin cories seem to be a bit more predatory upon their eggs than are other species, so the eggs should be removed, as above.

It’s worth mentioning that orange-fin cories seem to be among some of the most noxious cories when it comes to releasing toxins. These fish should be bagged individually and handled gently between the aquarium store and the home. A couple of pellets of activated carbon in the bag are helpful, as is a small piece of filter media.

C. Eques

The eques cory cat, sometimes called the knight cory cat, is one of the most sought-after species of Corydoras. The fish has a dark, almost black, body with an orange stripe along the back, sometimes over the belly, and onto the face. It is a beautiful, striking fish. It is often confused with a cory that has had the name C. venezualanus appended to it, even though it has been considered to be synonymous with C. aeneus (although I personally believe venezualanus is a valid separate species). These two fish are remarkably similar in appearance, especially as adults, yet the eques cory commands a far higher price.

Most eques cory cats are caught in the wild, and care must be taken to acclimate them to the aquarium. Live foods are often needed, although pellets will soon be taken without issue. This species is very gregarious and should be kept in a small group. Females can be told apart from males by looking at the fish from the top; the females are significantly more rounded.

Triggering the eques cory cat to spawn is similar to what is done with other members of the genus. However, they lay their eggs in the leaves of plants, typically near the surface. This can be mimicked in the aquarium by the use of a spawning mop or water sprite. The eggs should be removed, as the adults will certainly eat them.

C. Robineae

One of the newer cory cats to appear on the market, the flagtail cory cat has begun to be imported fairly regularly; hopefully, it will become a staple of the aquarium trade. These fish are golden or even reddish, with an interesting dot-dash pattern along the body, and a tail with several stripes through it.

Most of these fish in the hobby are wild imports, and shipments of this species, healthwise, are hit or miss. I’ve brought them home and had fish that were practically paper thin, while other shipments looked like they came from a breeder’s system. This cory cat will do best in soft, acidic water, ideally with tannins staining the water from driftwood, oak leaves, or the like. The substrate should be fine sand or gravel.

Flagtail cories are more difficult to spawn than most, though the usual cold water change seems to do the trick. However, eggs are not laid on the glass but in the sand or, sometimes, in plants. The fish seem to work up an appetite while spawning, and they almost immediately begin searching for the clusters of eggs. As the eggs are small and easy to miss, it is recommended to remove the adults and lower the water level. The eggs hatch in three to four days, and the fry absorb their yolk sac within two days after that. They may then be fed a diet of microworms, banana worms, vinegar eels, and the like, until they are large enough to take newly hatched brine shrimp, banana worms, golden pearls, and other small foods.

Countless Choices

With the large number of different Corydoras species that there are, many of which are available in the trade, there are countless types of cories, and describing only four species is barely scratching the surface. I’ll be sure to continue to look at the cory cats in future columns of “Bottom of the Tank.” This is a truly diverse group of fishes that every aquarist should keep.