Personally, I'd rather use aluminum sulfate and hydrated lime to change the ionic balance in the pond water than put topsoil into the pond. The potential excess nutrient problems associated with the topsoil is what I'd try to avoid.

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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).