A fish kill of that size with the weather Georgia has been having the last month makes me think of a lack of oxygen. Without sunlight, a plant breathes oxygen just like your fish. A large amount of plants or plankton in the water on a hot still night will suck a lot of oxygen out of the water and some dead fish is inevitable. If that is the case, I would suggest looking into and aeration system to keep the water moving and stop this problem.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb