08-22-2010, 12:34 PM
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In their habitat, the species of Corydoras occur in groups (shoals) of hundreds of the (same) species. In several areas, different species are found together, but there are dozens if not hundreds of each individual species within that collective group.
A group of five of each individual species appears to work the best. Several species can be combined in the same tank as long as there are at least 3 but preferably 5 of each individual species.
In my experiences over 15 years I have found that the species tend to remain together more than mixing. However, there are many times during the day when individual fgish, or two or three of the five will be on their own. And sometimes different species will be together swimming. But for the most part, they tend to remain together as individual species when resting.
As previous responses have indicated, and I agree, you should have a minimum of 3 of any one species; depending upon the tank size, you could have for instance 3 of one species and 3 of another and 3 of another, or 4 of one and 5 of another, or all the same species depending upon your preferences. Corys are fascinating little fish, and having different species if you have the space works very well.
They have a preference for water flow too; some species that occur in very slow streams and pools prefer quiet tanks and will take up residence as far away from a filter outflow as they can. Others that occur in streams with more flow will take up residence close to the filter outflow. By "take up residence" I mean that when they rest it is usually in these areas.