Mechanical filtration of single cell algae is very nearly impossible, hence the pond can look green and cloudy. This is where the Ultra violet light comes in. This type of light will make the algae clump together, making il large enough to be caught in the filter. You should allow 1 watt of UV per gallon of water as a bare minimum. Many koi keepers will multiply this amount by three, to account for more direct sunlight and stocking levels.