What is below the 60 inch mark? To properly seal the bottom of a pond, ideally you should have a 2' thick layer of clay, compacting it with a sheepsfoot roller in 6" lifts. That blue clay is good stuff.

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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).