I've had ponds for many years, mostly in the 12-1500 gal. range. I've tried to make what ever I could to help filter the water. I see all the comments on rock on the bottom and the fact it collects gunk and is not good for the fish. I have always had no rock on the bottom of the ponds and there was always stuff that collected down there. This last build I decided to do it with 1/2" size gravel all over the bottom and shelf's. Looked good when I filled it then added my fish, overtime the water came back to the green cloud we all know. So I read as much as I could find on pool sandfilter adaptations for the pond. Ended up with a homemade skimmer feeding into 4- 4"wide brushes then to a pump rated about 1200 gph. From there it feeds to another pump ( pool cartridge filter setup w/ no filter ) then to the UV lamp and into a rescued pool sand filter filled with poly strapping. Packed as much as possible in there. Now to the rock filled waterfall spillway. In the second day it was going from the green cloud to clearing up, after a week and ahalf I can see the bottom ( 2.5' ) down. You know what I realized after watching the fish for hours, they clean the gravel bottom and move it around.