STAGE 4 – TWIST-IN MIXED BED COLOR CHANGING RESIN DEIONIZATION CARTRIDGE. Aquatic Life Twist-In Color Changing Dual Deionization Filter Cartridge: Replace when the resin color changes completely. After the water is filtered by a membrane (not included), there is usually a small amount of Total Dissolved Solids (TDS) left in the water. In certain non-drinking water applications, it is important to remove the remaining TDS from the water. This is accomplished by filtering the water through a resin that is charged with Cation and Anion resins (H+ and OH-). As the resin absorbs the TDS, it will change color. When the entire resin has changed color, it is time to replace the DI filter. The new twist-in media cartridge design makes filter changes simple, easy and clean – no tools required and no mess! The twist-in cartridges can be easily changed in only seconds with just a quarter turn and without contact with used filter material. It is important the cartridges are installed post membrane filter to prolong the life of the Deionization (DI) cartridge. Working Pressure: Minimum 35 PSI, Maximum 80 PSI, Ideal 65 PSI.