Maybe set up a koi pond half way down that hill, with a bog filter set at the top of the hill? Then you can draw water from the big pond, run it to the bog for filtering/clarifying, then waterfall or stream down to the koi pond, then waterfall or stream back down to the big pond?
This would greatly increase the useful volume of water, while still allowing you to easily view the fish. You could also stock the big pond with some native fish if you wanted to, although you'd want to keep the population low so as not to overload the system with waste.
This would greatly increase the useful volume of water, while still allowing you to easily view the fish. You could also stock the big pond with some native fish if you wanted to, although you'd want to keep the population low so as not to overload the system with waste.