It sounds to me like the water doesn't have enough DO in it, and is too warm. The GSH with their noses in the aeration upwelling *to me* points that way. Are there any fish piping at the surface in the very early morning hours?

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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).