This seems to be the best suited product i could find. "Mr. Sticky's underwater epoxy" http://www.eclassifieds4u.com/toronto/ca-sticky-underwater-epoxy-adhesive-glue-ad-nlmkp60322">http://www.eclassifieds4u.com/toronto/ca-sticky-underwater-epoxy-adhesive-glue-ad-nlmkp60322</a> Ill look some ore for larger quanity size.
"It is as strong or stronger than silicone. It is also flexible like silicone and so will never crack. I used it to cement an acrylic back wall to the inside of an all-glass tank so the new acylic wall became a new water-bearing wall. Water never leaked to the second chamber. If you glue your tank walls' bottom edges to the top of the <span style="color: red">PVC</span> base as is proper and run an inside bead of additional epoxy like I did, I am confident it will work."