if you don't have a large fish load, a few "growbeds" of gravel would work swimmingly
build a few "raised beds" around the pond that will act as your biofilter.. fill with gravel, drain back to the fish tank - you could run the growbeds "constant flood" where the water continously flows through the growbeds a few inches below the surface, or "flood and drain" with a siphon, pump always running
i've got over 100 fish in an 8' swimming pool, using about 600 gallons of gravel for filtration, growing lots of spices (basil, dill, rosemary etc) and some ornamentals (jade plants)
