You could go sponge filters and air pumps, internal submerged filter, external canister, external trickle or constant drip-continuous water change. I have even seen modified HOB filters, the kind with biowheels, made to work in a pond. The HOB was placed in the pond on a brick on one of the shelves. The water level was then adequate for the HOB to draw water into the filter and the water flowing out was like mini waterfalls. It looked/worked pretty good. No need to worry about the motors being submerged, they are encapsulated.
I would Google it.