DI Filter Cartridge with Color Change Indicator. - Specially formulated mixed bed resin is Ideal for aquarium application. - Reduces TDS to ZERO - Place at final stage in your RO system - Purple color turns to light brown when resin bed needs changing - Refillable Cartridge Lewatit® NM 65 is a premium grade, gel type, high capacity, color indication mixed ion exchange resin prepared with a 1:1 chemically equivalent ratio of cation to anion resin. Lewatit® NM 65 is a mixture of gel type cation exchange resin and a standard cross-linked gel type anion exchange resin. The mixed resin is prepared from component resins which have been manufactured with the highest degree of purity and conversion to the H+ / OH form.