If it's a swimming pond, I'd cover the floor wherever feet will be disturbing the clay. How thick? I'd plan on 6" or so. If on a slope, a berm will be needed at the lower edge of the sand/gravel to help prevent slippage to the deepest part of the pond. FHM can live in shallow water.

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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).