Corydoras catfish stay small, help scavenge and play nice with others. Additionally, corydoras catfish -- or "corys" -- adapt well to a variety of water conditions. They can share a tank with most any fish that doesn't require extreme water conditions and won't pick on them.
Small Schooling Fish
Cory catfish have no problems sharing a tank with small, active schooling fish. Tetras, like the neon, cardinal and serpae tetras fit this bill. Additionally, livebearers like the guppy also will get along well with corys. On top of getting along well with cory catfish, small schooling fish usually hang out in the mid- to upper levels of a fish tank. This helps fill out the tank, since corys tend to hang out near the bottom.
Otocinclus catfish, also know as oto cats, come from the same South American rivers as corys. Like corys, otos tend to school, stay small and get along with other fish. A group of otos will contrast with corys and will not bother or pick on corys. Otos have the added benefit of being algae eaters. However, you will need to provide otos with vegetable-based foods, like veggie wafers, to make sure they get enough to eat.
Though the cichlid family is infamous for large, aggressive fish, there are plenty of exceptions. Small, peaceful cichlids like the ram and krib have very similar requirements to corys. Additionally, dwarf cichlids sometimes feel stressed and shy in aquariums. A school of corys can act as "dither fish," small fish that show the cichlids that there are no predators around. For this reason, dwarf cichlids and corys make great tankmates.
Most importantly, corys need to be kept in groups. In the wild, corys school for protection and will feel stressed out and vulnerable in captivity if they don't have others of their own kind around. Hundreds of species of corys are available, and many have very different patterns from each other. Always keep your corys in schools of six fish. Since corys don't have a territorial bone in their bodies, you can keep as many schools of different corys as your tank can hold.
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