Aquatic Life DI Color Changing Mixed Bed Resin Cartridge for Aquariums, 31 Cubic Inch Cation and Anion resins (H+ and OH-) remove the remaining total dissolved solids (TDS) from membrane-filtered water Cartridge is refillable to allow for easy customization of water treatment The resin changes from Blue to Yellow, and finally White when media is exhausted. 1/4" tubing easily attaches to the press-fit connections on each end of the cartridge