If it's a stationary pond, I would use a couple of raised-bed filters. I would construct stands for two halves of a 55 gal. drum, take a whole drum and cut it down the middle, so you have two half-cylinders.
I would drill a few drainage holes in the end of each, near the rounded bottom edge, raise the other end of the half-barrels slightly, and drop a coarse pebble into the bottom, perhaps with a
biochar layer sandwiched between landscape cloth, and topped with a
garden bed.
I would make a slight partition of landscape cloth at the raised end of each half-barrel, and fill the partition (not large, just enough for the purpose) with a sand-filter arrangement, into which a pump can deposit duck water without washing away the garden soil. The pond water
should flood the surface, but not so much that it overflows or displaces soil. A layer of sand might do well between the landscape fabric barrier and the topsoil.
I would plant heavier-feeding plants near the sand filter end, and those requiring less would go nearer the drain.
If the lower lipped ends are left to drip back into the pond, the water will be clean by the time it reaches the pond again.
If you want to grow plants that the ducks aren't to eat, why grow them where the ducks are going to be?
I wouldn't put anything in your duck pond that you don't want eaten. In fact, I would probably encourage the growth of duckweed. You could grow aquatic plants that ducks do eat, but screen off the main plant so they don't kill it completely, but can easily eat anything that pokes out of the protective screen. In that way, the plants will grow back, to be cropped back again by the ducks.
As to fish, I would add some white cloud mountain minnows (I think that's what they're called) to combat mosquito larvae, or anything else that you don't want overrunning your tank. I would also get a bottom feeder. My choice would be a
local small catfish, brown bullhead, but you might have other choices available. Koi might actually do the trick, if you're uninterested in eating them.
You might want to consider making a long sock-like arrangement to contain some kind of growth medium, even silt and muck from a natural pond local to you. This would be hung just under the water level around the inside rim of the pond, and you would want to plant a local reed bed guild in the sock. It might require a little additional protection, but you could have a ring of reeds around your duck pond, giving them
shelter and seclusion from predators, and the reeds would actively filter the water, reducing the load on any filtration method you choose that requires a pump.
There are many options. All you need do is tailor an approach to your specific situation. Let us know how it goes, and good luck!
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein