Dissolved O2 starts dropping when the sun goes down, and is lowest before sun-up. Did you check the DO during the middle of the day, or early in the morning? Hotter water holds less DO. Larger fish go first due to higher DO requirements than the smaller fish.

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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).