No expert here either, but have a theory to ponder. If you aren't keeping up with or exceeding the O2 consumption rate on the bottom, then the water you are bringing up is "bad water" as in low O2 and hydrogen sulfide laden. If you are constantly compromising the top water where the fish want to be, that could be badness. In that case, a significantly undersized aeration system might not break up the thermocline, and not push through to a full circulation where water coming up is decent. The little O2 going down is consumed and levels are still not enough to support aerobic bacteria or fish. Then aeration may just continually bring up more bad water and fish are worse off.
