The slow run of the water really helps. Our veggie filters are 4 x 8 x 1.5 feet and the water cycles through in 45 minutes. The result seems to be pretty good. The roots of the veggies do their thing and virtually no sediment goes back to the pond.

I think the key for us was enough plants and filter time. Algae competes poorly with vasculars. You can see it begin to 'rot' when the plants get going.