Personal experience here. I think not much organic growth is forming on the plastic is because the lake is infertile. I've put white PVC pipe in a vert fertile pond (visibility 12"-18" max.) and after a week the PVC was already coated. I did NOT sand it at all, as it was plumbing for a waterfall feature.

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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).