Doulton W9125030 fluoride reduction replacement candle is a 10×2 Doulton ceramic filter that can be used in DUO system. This filter cartridge reduces the fluoride in drinking water. It  is designed for gravity fed filters in order to remove fluoride. Head mount: 3/4 inch (19 mm) Capacity: approx. 3 months Flow rate: 0.4 gpm ( 1.2 gpm @ 3.0 PSI , 5 gal per 24 hours per candle in gravity filters ) Pressure 10 – 100 PSI