I doubt it is your aeration. At 85F the ability of the water to hold DO is drastically reduced. No matter how much you pump in, water at 85F will max out at about 7ppm, in optimal conditions. It is hard to believe, with your water temp and fish kill, your ammonia is at 0? However, it is looking more like a DO issue.
Are you feeding the fish? Can you get a better picture of the pond scum?
In your case, I would look specifically at gill color and blood color. Red/Lavender gills may indicate high NH3 and brown blood would indicate high nitrates. Also, fraying of the fins, is a sign of high ammonia. Catch a live one and compare the necropsy to the autopsy.
https://akffa.files.wordpress.com/2013/02/performing-a-fish-autopsy.pdf
