Mike-As Bill mentioned i learned the same way you did from the school of hard knocks. The bottom diffuser is fine to run in the winter at night to keep the ice off, but to run in the summer is a disaster. You and i had the same exact problem. The bottom diffuser mixes the water colume and will not provide a cool refuge for the trout. I lost 160 Scam Trout the same way. I solved the problem by installing a Kasco Top water aerator and run it at night and on cloudy days. When i installed it the fish immediately left the surface and i didn't lose a fish again due to oxygen. I also run well water during the hot days to keep the water on the deep end cooler. Sorry you had to learn the hard way but thank god for this forum. You can find Kasco on the Internet. Expect to spend some cash to get a good one.