Don't forget about the Hydrogen Sulfide that is also mixing in the water column from the anoxic bottom layer. If that is abundant enough, it can kill the fish even if there is enough O2 in the water, and it doesn't take much H2SO4at all to kill them.
Thank you Scott. My tired old lazy brain couldn't remember the term
Hydrogen Sulfide so I skipped it when I wrote that.
Anybody who has walked around in the muck of a pond or swamp, and got the rotten egg smell, has experienced the effects of H2SO4.
