I just had the same thing happen to me about two weeks ago. My pond is 1/2 acre and a little over 30 years old. After one of our recent heavy downpours. It took me three days to remove all the dead fish. In total about 30 gallons of fish. My water went from clear to less than 8 inches of visiblity with a lot of black globs floating. I do not have a farm runoff. I do aerate but have only been 8 hrs a night with one diffuser. I will be installing a second one shortly to keep it mixed up.
I have not seen any signs of survivors. Only tadpoles. I have set out a fish trap but have not caught anything yet. This weekend I will search for signs of life.
I might take this opportunity to go from BG and LM to YP and SMB.

GKLOP