Have the catfish deaths stopped? Catfish are documented territorial - pecking order fish and to minimize fighting in confined spaces they have to be held at certain densities per cubic foot. Possibly your stocking densities for the catfish are too low.
Catfish can be bottom feeders or feed pelagically, but they do not eat sludge and dead aged materials comprising the bottom sediments. "Bottom feeding" most often means feeding on living organisms associated with the bottom areas. In the case of omnivores some of the consumed materials can be fresh dead materials usually animal origin and occasionally plant materials.
