Twin Value Pack 18 Watt UVC UV PLL 4 Pin. PondH2o UV bulbs are designed for UV-C Filter & Clarifier systems for both Ponds and Aquariums. Value Two Pack so keeps your filter effective for a full 12 months. Germicidal lamps operate at 253nm to greatly improve the efficiency of your filtration system when water passes by the UVC lamp. PondH2o UVC bulbs work to clear your pond or aquarium water by removing unsightly green algae . The strong ultraviolet light breaks down the algae in the water. For optimum UVC effectiveness, Twin Value Pack 18 Watt UVC UV PLL 4 Pin should be changed every 6-12 months. Please make sure that you are purchasing the correct 4 pin UV Bulb by checking your filters instructions for the desired bulb wattage and length.