Ok with that said sounds to me that you need to keep some fish and have a fish fry!

With D.O. issues they say the bigger fish will die off first... So sorry to hear that man!! Sounds to me like with the heavy rains and no air in the pond you may have had a small turnover issue? Or some low D.O. areas in the pond and the fish got caught in it?
I am no expert on this by any means, but sounds to me like to many fish and not enough D.O. to go around maybe? Lets see what other have to say. How deep is your pond?
RC