A UV light in a pond helps to combat unsightly green water algae. We recommend replacing your fish pond uv bulb every 6- 12 months to maintain optimium performance. TIP: Change your lamp at the beginning of every pond season(March to April) to ensure the bulb is at full strength during the warmer sunnier months. This 18 Watt UV light bulb is compatible with UV units from tops brands such as Hozelock, Fish Mate, Blagdon, Jebao and any other pond UV or filter system that requires a 18 watt PLL tube/lamp