DuoClear® W28001 Single Mineral Cartridge 35K gals 4/Pack works automatically. So you can spend more time enjoying your pool and its fresh crystal clear water. All you have to do is replace the DuoClear® cartridge every six months. DuoClear® even has a signal light that reminds you when to do that. Zodiac's DuoClear® system makes pool water simply irresistible. The DuoClear® system uses silver and copper in conjunction with just 0.5 ppm of chlorine to destroy bacteria and algae. The silver is an effective bactericide, while the copper is used to combat algae. DuoClear® produces low levels of chlorine to oxidize dead bacteria and algae. The Zodiac DuoClear® system saves you the time and effort that comes with manually sanitizing your pool. This system works to give you the best quality water possible. It's the clear choice for maintaining perfect, quality pool water automatically.