When the pond is being dug out, if you run into good clay, I would plate over the rock with clay and tie it into the clay in the rest of the pond basin.

If there's any cracks in the rocks and they aren't sealed, water will leak out. If there's more water coming into the pond than is leaking out, you'll never notice the cracks. But if not................

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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).