Donnie - It sounds like a dissolved oxygen problem to me. Bob Lusk recently wrote that practically all summer fish kills are dissolved oxygen related, but I cannot locate that quote righ now. The fish kill definately was not a "too much heat problem" or else just about everyones fish in your area would be dead. Neighbor's ponds will be a similar temperature as yours and they likely do not have lots of dead fish. However high air/water temperatures probably played a part - see below.
Your situation is a good example why a bottom aerator is not 100% effective at preventing a fish kill due to a dissolved oxygen shortage. There are special or unique situations where certain pond conditions will even overwhelm an aerator and a DO loss occurs. Dissolved oxygen does not have to drop to 0. A DO decrease below 3 or 4 ppm (mg/L) will cause fish deaths. Small fish tolerate low DO better than larger fish. Catfish and especially bullheads have highly vascularated air bladders and can obtain a small amount of oxygen from atmospheric air by gulping air at the surface which is no doubt what the catfish were doing when you saw them at the surface.
Numerous situations could be the cause for the DO shortage. 1. Aerator is undersized. 2. Aerator is only mixing a small percentage of the pond and a section of the pond or unmixed region will lose DO and a partial fish kill occurs. Partial can be any amount from 1% to 98%. 3. An algal bloom grew out of control and too much algal and or rooted plant biomass developed. This over bloom problem often happens in fertilized ponds. This in combination with very warm water, that does not hold a very large amount of oxygen even at 100% saturation, can result in not enough DO to sustain the fishery during the entire dark period. 4. A heavy algae bloom crashed i.e. water went quickly from greenish to clear or brownish. A crash of an algae bloom quickly absorbs a tremendous amount of DO esp at night; more than a bottom diffuser can replace during darkness. 5. Weather conditions such as one to several cloudy days with a heavy bloom causes less than optimum production of DO during daylight and DO becomes too low at night. Again, this usually happens in real warm water temps.
Your pond could have developed a toxic algae bloom but I seriously doubt this since you are in KY. I have not read where KY waters are experiencing any outbreaks of toxic algae blooms.
Let us know what your visiting fish biologist has to say after his DO test and visual examination. Note - DO conditions in the surface and near surface waters can go a long way toward recovery after 24 hours and a day or two of sunny weather. It all depends.
