Thanks, all. The 2x4 idea was one way I might go, but I was thinking it might look better if I found some pavestones that match the patio and use those to hold down the liner all along the edge. As for plumbing, I'm just going to run a flexible tube from the pump through that gap that you see in between the house and the inside wall of the large box. In terms of what is going to live in there, I think I'm going to use it for seasonal, tropical fish, (either breeding colonies of cichlids or livebearers to make babies that I can harvest or maybe just a summer home for some oscars that I can move back inside for the winter) but this year, I'm going to use it to grow out some baby koi that I will put in my parents' much larger, in-ground pond when it gets cold. I think that this pond is way too shallow/exposed to use for any fish that would want to over-winter in the pond (eg goldfish/koi) and I'd be nervous about the ice formation splitting open the wood, so I think I'm going to drain it and cover it for the winter months.