I wonder if contact-stick clear film applied to the water-side surfaces would be cost effective? Need to clean? Peel it off and replace.
I love the inverted aquarium idea in a garden pond though. I'm not sure if a PVC-based frame is sturdy enough, and metal could puncture a liner leaving wood as a reasonable choice. You also have to be careful of loosing water level or waves in wind and breaking the seal.
Some underwater lighting pointing up into the inverted aquarium at night would be pretty cool.
