Thermacline at Coan Lake
This time each year the water has a "Turn Over" where the water temp turns from warm to cool (flip from bottom to top) which removes oxygen from the water and the weaker fish perish. The technical term is called Thermacline - which on big bodies of water has little impact on fish but on small bodies of water it can be devastate the weaker fish population. Coan Lake water has been tested and it came back very clean so there doesn't appear to be a water quality problem.
If you would like to know more, check out the article from Lake-Aeration.com