Last year's drought dropped the water level down to 3 or so feet from 9'. We had ice storms which added to the ice on the pond's surface. When the ice started melting, there were dead fish embedded in the ice and many floated to the surface once the pond was completely melted.
A fish kill happened years ago but not as complete. Part of the blame for low oxygen may have been the 4 grass carp that had grown to 8 lbs. that ate too much bottom vegetation. They of course were floating with the rest. Also, there were many spawns over a decade from bass and panfish possibly leading to overcrowding.

I don't expect those conditions of drought, too little vegetation and thick ice to exist in the near future, but is aeration an option? As is, I'm holding off on putting in grass carp unless bottom weeds get too much, but have restocked with a few bass and many panfish. A spawn did take place as evidenced by a fingerling seen yesterday..

It was so sad seeing all those dead fish, but heartening catching fish for the first time since last fall. Usually, sunnies would swim near me expecting small bits of bread thrown to them. None so far - only one small turtle (3 of 4 never returned.)