Mom has a 10' circular stock tank buried off her back porch set up in a similar fashion. A few water plants (plus garden all around) and the pump do it. She does clean out leaves, hickory nuts, etc. every so often though. Even then, with the pump turned off in the winter (help water temp stabilize) the water stays clear and OK. She keeps gambusia in it, and stores feeder goldfish for the herons, racoons, snakes, etc. that come by for a snack.
