esshup, thanks for typing up all of your personal data and also for cleaning up the thread.

I wasn't quite following your point in your original post. I think I see it now. However, it appears to me that you are conflating two different issues. The first is reducing the pond temperature during the hottest time of the year, and the second is running optimal aeration for the pond during that period.

I have zero experience running temperature profiles on ponds IRL. I will just throw in my two cents worth on a theoretical basis.

A stratified pond will have a much cooler AVERAGE temperature in the summer than the same pond that is overturned completely due to aeration. Both ponds would have almost the exact heat gain during the day. However, the stratified pond would lose much more heat during the night, resulting in that pond staying cooler over the course of the summer.

The higher heat losses in the stratified pond are due to the hottest water being at the surface of the pond. Radiative heat losses are a temperature dependent function. The hotter water will radiate out more heat at night.

Perhaps even more importantly (based on conditions), the stratified pond will also generate much greater evaporative heat losses at night. The energy input to turn a gallon of liquid water into water vapor is much greater than the energy input required to raise the temperature of a gallon of water from 32 degrees to 212 degrees. In many circumstances, evaporative heat loss is a very important factor! The longer at night the stratified pond can keep the air temperature at the surface interface above the dew point, then the longer the pond will shed large amounts of heat due to evaporative heat loss.

Applying that theory to your observations does provide an explanation for both of your scenarios regarding the trout. For the pond that you aerated only at night, the pond lost its stratification and therefore became warmer than it would have without aeration. Your trout did better in the stratified pond WITHOUT aeration because it was cooler. However, if you had operated surface aeration on that pond, then the average pond temperature would have been cooler AND there would have been more oxygen in the surface layer. I suspect those trout may have been able to live a few weeks after July 4th.

Likewise with the second pond. Moving the aeration off of the bottom allowed the pond to stratify (cooler total water) and the well oxygenated surface water enabled the trout to survive all summer. However, if you had made those aeration changes AND only run the aerators at night, then the trout may have had an even more favorable environment due to the cooler average temperatures and the higher dissolved oxygen level at the thermal refuge where the trout could hang out just above the anoxic bottom waters.