Laying down thick geotextile fabric over the cinder blocks and under where the small rocks will go might mitigate the roughness of the blocks and stop the small stuff from disappearing into the dirt on the bottom of the pond.

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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).