I'd say you have an anoxic layer coming up probably brought up by a thunderstorm or a rapid drop in air temps. If you would have had a bottom diffuser in place, instead of surface aeration, you wouldn't have had this problem.

Are the fish hanging around your surface aerator?

99 percent of fish kills are due to low oxygen due to various causes.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.