Deionization (DI) Resin is used as the last stage of a RODI system to reduce any remaining total dissolved solids (TDS) down to undetectable levels, making perfect water for reef tanks. Professionally Packed Color Changing (Changes from blue to brown as it depletes) Vacuum packed in foil-lined 4mm Mylar bags for a long shelf life Includes Refillable 10 Cartridge Nuclear Grade Pre-Packed DI Cartridges let you just remove and replace your current DI cartridge without having to repack it with new resin that takes time, and can create a mess.