Sealing a fish pond

Why not just use a rubber pond liner material? Heck, my neighbor made a small pond and used two layers of 6 mil black plastic.

I had a Koi pond for two years, but it was a maitenance nightmare. Not because of the fish or the pond filtration system. It was because I put it in the front entry atrium of my house. It caught all the leaves and run-off water from two roof slopes. What was I thinking when I dug that hole?