It seems like my pond is being overpopulated with catfish. Will the big ones eat the little ones? If not, what kind of fish can I put in that will?

sometimes catfish seem to bite/eat eachother, i got few recently with chewed down tail, or worse...

probably some real predator like snakehead, or barramundi would do the weeding for you.

My wife put in quite a few snakeheads but we never see them. Maybe the bigger catfish ate them. ??? The pla nin have been in the pond the longest period of time but they are still small despite the fact that my wife gathers and puts all sorts of water plants in the pond. The plants apparently get eaten before they can establish themselves. There were quite a few plants growing in the pond before we stocked it with pla nin. Since those plants are gone the water has never been as clear as when they were growing.

sneakheads might not be visible and still be there.

i never caught or really seen any in my lake ( the one i used for fun fishing) but found quite few when recently drained...a good many 2-3 kgs.