Don't use "foam core" PVC pipe. It will float in water. I went the cheap way some time back; and bought some to make crawfish habitat, then after it was made I realized that crawfish wouldn't use it if it was floating.............

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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).