Corydoras catfish are commonly kept in freshwater aquariums. They are from the genus Corydoras and the family Callichthyidae. There are several different species that are all similar in size, body shape, and behavior. However, the different species have different coloration and markings. In some species their markings are quite striking.
They are often referred to as armored catfish because they have two rows of bony plates on each side of their body. The upper part of their mouth has 2 barbels, which can be seen in the above photo. The barbels are used to help locate food and sometimes for feeling their way around the tank. They usually reach a size of between 2-3 inches or 5-7.5 cm. There is one species that is very small even for a corydoras. It is the dwarf corydoras (Corydoras pygmaeus). It reaches a size of only 1 inch or 2.5 cm.
Corydoras catfish have a labyrinth organ that allows them to breath atmospheric oxygen. In fact, they must have access to oxygen from both the air and from the water (they also have gills) or they won't survive. Although corydoras are bottom dwellers you will frequently see them come to the surface for a gulp of air.
They are excellent fish for community tanks. They are very peaceful and they never bother any fish that isn't small enough for them to swallow. They also do a great job of cleaning up excess food that may be left over after feeding your other fish. They are always searching for food along the bottom of the tank.
Corydoras like to congregate together in the aquarium. They usually hang out together while searching for food and they even rest together. You will often see one catfish resting his head on the body of another catfish. They don't even have to be the same species to get along and become friends. They only need to be of the genus Corydoras. Because of their social nature, you should not keep only one corydoras in your tank. You should always have at least two, and 3 or more is even better (as long as you have the room in your tank). Corydoras don't survive long alone.
Corydoras aren't too picky about what they eat. Their mouths face downward and so they feed off the bottom of the tank (mainly). Sinking foods are good for them. They will also eat flakes or other fish foods that have sunk to the bottom of the tank. They will also eat freeze-dried or live foods, but usually only if this food is found near the bottom of the tank. They will sometimes come to the surface to eat freeze-dried worms, but sinking foods are best. I often stick a cube of freeze-dried worms on the glass near the bottom of the tank to feed my corydoras.
Corydoras do best in tanks that have already been cycled. They do best at a neutral pH (pH of 7.0) and the water temperature should be between 70-79 degrees Fahrenheit or about 21-26 degrees Celsius.
The bottom substrate of your tank should be either sand, or round rocks. If you use rocks with sharp edges as your substrate it will damage their barbels. Also, corydoras generally don't like added salt to their tank. I have added small amounts of aquarium salt when I have been treating the tank for a disease (such as ich), and the corydoras have survived it, but I always do a partial water change a few days later to remove some of the salt.
Sexing corydoras catfish isn't always easy, but the females usually have wider bodies than males do. They usually spawn in groups of 3 (1 female and 2 males). They usually assume a T position. Before releasing her eggs, the female corydoras puts her mouth against the male's genital opening and swallows some of the sperm. This passes through her digestive system unchanged very rapidly, and onto the eggs she has released and is holding in her ventral fins. After the eggs have been fertilized, the female corydoras carries them with her ventral fins to a flat surface in the tank, such as on plants or on the glass, where she sticks them. They hatch a few days later.
In a community tank it is likely that the eggs may be eaten before they hatch. It is best for spawning to occur in a separate breeding tank. After spawning is complete the parents can be removed because corydoras don't take care of their fry.
Once the fry hatch and are free-swimming they will need to be fed. Remember that they are bottom feeders and they will need foods that sink to the bottom of the tank. At first they can be fed infusoria, liquid commercial fry preparations for egg layers, and when they are a little older, newly hatched brine shrimp.