There is a company in Oregon called bend tarp and liner. They make a tarp for Koi ponds. They actually use it to line lakes and large decorative ponds in golf courses, etc.. It is really inexpensive, I think 32 cents a square foot retail. It is water proof and won't break down in the sun. You can install bulkheads and bottom drains and it is much lighter than 45 mil EPDM. The only draw back I can see is it is harder to seam your folds. Other than that it is a better product than EPDM because it is very difficult to puncture or tear. Just some thoughts.
