I had to look up "mesotrophic," but that just means not too low or not too high as far as nutrient load.

It doesn't explain the fish kill you had.

The kill may have been due to the blue/green algae, or the aeration bringing up some bad water, or some other reason.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."