It used to be I couldn't keep cory's. They died for unknown (at the time) reasons. Then I started keeping a very clean tank, with no substrate, and they thrived. I've since moved them to a tank with pool filter sand, which I keep super clean, and they are fine. Looking back, I think the dirty gravel they laid in most of the time was the problem. If you keep a clean tank and substrate, they are happy. I also found that they love to swim long lengths of the tank, so a 55g where they can go from one end to the other keeps them active and happy. In smaller tanks, they just hung in corners, not really swimming much. Open space keeps them happy I think.