Pond Boss Forums INTRO Questions & Observations Fish die off.... Something I need to look into more??

A batch of deaths after a major, hot Summer rain event sounds like you may have had a turnover - colder, less oxygenated bottom water mixing with the better aerated surface water, resulting in lower than optimum O2 for the fish.

Any marks on the dead BG that you've been finding since then? I can't recall hearing of any delayed deaths from a turnover, but I may have missed that aspect.

Your bottom water pumped up to the top probably does not aerate as well as an air diffuser system would. If the pump will produce sufficent pressure, I bet you would be ahead to squirt the discharge out horizontally just under then surface, horizontally aerating the pond and rotating ALL the water in it up to the top regularly. See Horizontal Aeration.