Over the years I have had several tanks and many kinds of fish. My favorites are the Betta and the Corydora. My next project is a South American biotope with only Cories. How many can I put in a 20 gal. long tank? Also, I read in a book about filtration that a reverse-flow undergravel system is recommended to reduce the amount of bacteria in the substrate which can damage their barbels. Wouldn't the upward flow of water disrupt their eating habits at the bottom? Any ideas? Does anyone have a recommendation as to what filter is best for catfish?