I'd bet on a low oxygen problem. Wind will create waves, and they will add O2 to the pond. I agree with CJ, get something in there to beat the surface water to a froth - the only way right now to add O2 to the pond is by mixing it with air. If you had a Micropore diffuser you could run to the welding shop and rent an O2 tank and gauge.

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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).