The Rusco SS1 Sun Shield inhibits algae growth inside the Rusco water filter system cover, which can grow inside clear covers if sunlight is not stopped by a sun shield. The Rusco SS1 Sun Shield only works on 0.75" and 1" Rusco Spin-Down Filter Systems and Vu-Flow 1" Screen Filters. If you have a 1.5" Spin-Down System or a 0.75", 1", or 1.5" Sediment Trapper System, you must use the Rusco SS1.5 Sun Shield.