Catfish can handle low oxygen levels better than most other fish species. I have a pond about the size of yours, and have had as many as 700 catfish in it without a single loss due to oxygen
depletion. And, I have run only a small aerator for the last couple of years. My guess would be disease or something toxic in the water. I know that acidic water conditions can be deadly for catfish. Of course, I'm only speaking from my own experience and certainly not as an authority on the subject.