by Paul Schuman (originally written for the Honolulu Aquarium Society )
Corydoras are one of the most popular, if not the most popular group of aquarium catfish. Their small size and gentle nature has made them popular with aquarists for over a hundred years. They were first introduced to the hobby in Europe during the 1880s, and their popularity has continued to grow. They won't bother the smallest, most delicate species, and have armor plates on their body and stiff spines to ward off more aggressive fishes. They can be kept with most fish, except the most aggressive ones, and those requiring brackish or extremely alkaline water.
There are between 130 and 140 known species of Corydoras . All of them come from South America. They range from Argentina to Colombia, with most coming from the Amazon basin. They range is size from about 5 inches (C. barbatus) to about 1 inch (C. hastatus and C. pigmaeus). Most come from fairly soft, acidic, virtually salt free waters, but will adapt to harder, more alkaline water. Corydoras are able to absorb atmospheric oxygen through the capillaries in their intestines. Periodically, you will see them make a mad dash to the surface of the tank for a quick gulp of air, which they swallow. Occasionally, you will see them excrete bubbles as the air passes out of their intestines. This ability to utilize atmospheric oxygen allows Corydoras to exist in water conditions that will kill other fish. This doesn't mean you should allow your tank's conditions to deteriorate though!
Contrary to what you may think, Corydoras do not eat fish waste! They need good quality foods, just like the rest of your fish. I feed mine flakes, sinking tablets, live tubifex worms, live brine shrimp, live daphnia, frozen blood worms (midge fly larva), and frozen brine shrimp. Regardless of what you feed, make sure some of it is getting to the bottom of the tank. Otherwise, your poor catfish will go hungry.
The most widely available, and easiest to breed species are C. aeneus and C. paleatus . Both species are available in an albino form. Either of these species is a good choice for your first Corydoras. They are extremely hardy, and they have been tank bred so long, they will adapt to practically any water conditions. They have been kept in soft to very hard water and pH ranging from 6.0 to 8.2. They will live in temperatures of 68 to 82 degrees, as long as sudden changes are avoided. I've read accounts of them surviving in salt contents of up to 2 teaspoons per gallon, but I wouldn't recommend this.
Once you have mastered keeping and breeding these "easy" Corydoras, you may want to keep an eye out for some of the more rare and difficult species. It used to be difficult to find different species of Corydoras. Other than the C. aneus and C. paleatus, you could usually find C. trilineatus (often misidentified as C. julii), C. punctatus, C. melanistius, and occasionally C. arcuatus. There weren't many others available. The situation has gotten much better in the last few years. I know in the Honolulu area, there are currently at least 20 species of Corydoras available. I believe the increasing popularity has allowed importers to locate and ship species not formerly imported.
The challenge of these new imports is getting them to spawn in the aquarium. Many species haven't been bred in the aquarium, and demand a high price. I try to make my water conditions as close as possible to those where the fish come from. Often, this requires some research, to find out which rivers they come from, and what the water conditions are in their home range. Duplicating these conditions is sometimes enough to get them to spawn. Other tricks I use are waiting until the South American summer months, when the rainy season starts, and doing a massive water change with distilled water. This water should be several degrees cooler than the tank water. The idea behind this is to duplicate the rainy season, when most Corydoras spawn. If this doesn't work, Try waiting until a storm hits your area, then do the water change. Sometimes the drop in barometric pressure, along with the water change will work. I also use wood in my tanks, to add tannic acid to the water. Many Corydoras come from waters high in tannic acid. You could use blackwater extract for the same effect. Often, I can't get wild caught fish to spawn despite my best efforts. This is just part of the fun (or frustration) of keeping rare Corydoras.
I've covered the basics of keeping, feeding, and breeding Corydoras. If you need more information, there are many good books available that will give you detailed information on the particular fish you are interested in. I hope you get as much enjoyment from keeping and breeding Corydoras as I do.