anthropic, read this article. Hydrogen Sulfide While it is aimed at aquaculture, it is also related to your pond. While a turnover reduces the amount of dissolved O2 in a pond, it also mixes Hydrogen Sulfide and small amounts are toxic to fish. That's why there is a slow start-up with aeration systems in ponds that are stratified.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).