Peter (!),

We went with the 45 mil roofing EDPM. It is a decade old and doing
very well. The 'certified' pond-safe, expensive pond liner is the
same thing. If you wanted, you could wash it off. No one that I am
aware of has noted any ill effects among their fish from the roofing
liner. Seams can be made as needed. Be careful with them.

Have fun.

Jim