Deionization Resin (DI) is the last stage of the RO/DI system and is necessary to reduce Total Dissolved Solids (TDS) to zero. Bulk Reef Supply carries our own BRS brand of DI resin in regular and color changing. Color changing is more convenient for replacement as the color changes from Blue to Brown when the media is exhausted - removing the guesswork. Available in One Cartridge Refill or 1.5 Gallon - both vacuum-sealed for maximum quality preservation. Clear refillable cartridges are available in a variety of sizes - 2.5" x 10" is most popular.