After I sent my previous post I remembered more clearly what the marine sanitation company had said regarding custom-made plastic tanks. As the tank is being fabricated, both sides of each joint are accessible for welding so the seams will be very strong. The problem comes when the last side--- usually the top--- is installed. Once the top is in place the fabricator cannot get to the inside of the seam. So he can only weld the outside of the seam. This is why the seam around the top is weaker than the rest of the seams, and they have seen cases where this seam has let go on a filled-to-capacity tank with unpleasant results. ______________________________ C. Marin Faure GB36-403 "La Perouse" Bellingham, Washington