By simply twisting a knob on this filter it automatically cleans itself! In the \'run\' position the water undergoes a 4-stage purification process. Simply turn the knob to \"Clean\" and the foams are squeezed by up to 1,000lb of water pressure forcing out pond sludge. The back-flushed dirty water is then pushed out through the second waste outlet (typically down the drain or flowerbeds). This unit boasts a built-in 13w UVC and guarantees clear water for ponds up to 15,000 litres/3,000 gallons. (Figures based on pond in full shade with waterplants - 7,500 litres for ponds in full sun). NOTE: UVC bulb included.