I like the idea of fresh water rays better also. While I've never kept fresh water rays myself I've kept saltwater rays, blue spot and Cortez. Not easy to keep for sure and at the time I had kept, bred, propagated quite a lot of salt water stuff. You'd be able to build a small "room" at one end of the pond to house the filtration with a waterfall to return to the pond. You could grow quite a few plants in the waterfall area that would look tropical and help remove ammonia and nitrates. Think monstera, small palms, pothos, and some ferns. The "room" could be covered in the fake rock. With salt water your filtration would be harder.