The sun shield fits over the clear filter cover in order to block sunlight. 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. If you have a 2 Spin-Down System or a 2 Sediment Trapper System you must use the Rusco SS2 Sun Shield.