Filtration consists of two basic parts,
removing the solids from the water then nitrification/aeration.
Waterfalls help aerate the water, good for fish...but will do nothing to remove the solids.
Think most folks dump the duck swimming water, using smaller vessels for ease of drainage.
Removing poop from a pond is not an easy thing to do,
I learned this when studying/building ornamental fish ponds,
but it's imperative for clean(er) water.
No plants nor fish will help, they both also contribute to 'dirty' water.
A pump(poop pulverizer) isn't going to help much either,