this may sound stupid but cut the fish open and check for fire ants. its not uncommon around here for ants to wash into a pond and get eaten up by BG. they die from the bites they take internally. also check the gut cavity for parasites, environmental conditions may have caused an outbreak. not much you can do about parasites. stocking shellcracker help keep snail populations down which play host to many pond parasites.

drop me a personal email and I can shoot you over a autopsy based condition assessment paragraph from one of my books.