Dobber,
A few years ago I had a similar problem. I almost lost my lake. I thought that I had over fertilized. My lake turn soup green and I just got lucky that all the fish didn't die. I couldn't figure it out. Then one afternoon I was sitting in the den looking out across the lake and thinking how beautiful it was to watch the hundreds and hundred of white egrets flying low across the water and roosting in the dead trees at the lower end of the lake. THE LIGHT BULB CAME ON !!! I ran the egrets off and the problem
has never happened again. Just think how much poop that 1,000 egrets were dropping in the water every night. People should be aware that poop can kill a lake.