The white patches are likely to be a fungal or bacterial infections. Live fish will develop these infections when a stressor is introduced. As mentioned, aged fish are more suceptable.

Remove the stressor first. Your water got pretty hot if it was in the upper 80s. The water at the surface can easily get that hot, but if the water 18" down gets that hot... Your pushing your luck. When the water at 18" down starts to get in the 80s, I start shifting my aeration to night time only. I can do this because my system can still turn the pond over at least once in a 5 hour run time. Running at night helps to keep the water temps down. One diffuser in a half acre pond may have trouble doing this... Your system is probably too small for nighttime only aeration, but it should be looked into to verify. How hot did the main body of water get?

A massive influx of water could be a stressor, but for a month to go by lessens the likelihood that it was the main cause. Although, it is possible for the infections to have started then and added stressors influenced the fish's demise.

The numbers of deaths you mentioned don't sound horrible and, I agree that they looked to be aged fish, if not a little under weight too.

Do your research on a salt application. I believe that to be a good action to take, but not one you want to perform on a regular basis.

Fish on!,
Noel