Sorry for you loss.

Shallow water - big rain - high temps - my guess is a DO crash. Yes it can happen even with diffused aeration. That is why paddle wheel aerators are used in catfish farms. Diffused aeration is a great method which I support fully. It is not an emergency aeration system - it is not designed for that. When most of the plankton/plants go down in a DO event even a properly sized diffused aeration system can't cure the effect in the few short minutes it takes. They can and often do provide areas of safe haven which fish realize and move toward. You probably have survivors. High fish densities also contribute to situation. Find what caused the plankton to suddenly die and that is most likely the cause. Plant material washing in from a large rain event also contributes to the O2 depletion.