I have a very small pond, but have three very large koi that grew from 3 inches to about 24 inches long...Have had them for almost 5 years now. We use a UV filter plus a mechanical/biological filter. The UV filter really keeps the water clear and you can truly tell when the light needs to be replaced, as the algae growth increases rapidly. We use a box-type filter that is stuffed with a combination of foam-filled and carbon-filled balls, plus two layers of flat mechanical filters.
With three big fish (plus a pecca that is a bottom-feeder), the filters need frequent cleaning, but they really work.