18W UV Sterilizer & Polystrand Pond Filter. A combined multi-function pond filter of 154 Square feet of polystrand (in dual densities) around a high efficiency 18 watt Ultraviolet Sterilizer for superior algae and green water control. This filter handles ponds up 1280 gallons and is set in a modular format for adding on multiple filters. The UV sterilizer is UL listed and approved for outdoor use.