Green water, caused by millions of suspended microscopic algae, is the curse of many pond keppers, so what is the best way to control this nuisance? A good quality pump, filter and ultra-violetclarifier system, of a capacity to suit the size of the pond is all that is required. As water is pumped through the clarifier, the single-celled algae are exposed to high levels of UVC radiation emitted from a UV Lamp. This UV exposure destroys the algae cells, which are then broken down by bacterial action in the filter. The UV output of the lamps degrades with time, and so they weill need replacing periodically. This Hozelock UV will fit all 13W Hozelock UV products.