Coralife Pure Flo II Reverse Osmosis Mixed Bed Deionization CartridgeUsing resin as a water filtration media provides similar results to using a reverse osmosis membrane. Rather than filtering out pollutants as water passes through a membrane, resins work by attracting either positively (+) or negatively (-) charged particles found in water. The Coralife deionization (DI) canister provides a final DI filter step using mixed-bed resins. One drawback is that resin does exhaust and will need to be replaced or regenerated more frequently to maintain high quality water production. To help you better know when to change these filter cartridges, they are designed to change color from tan/green to purplish/blue when exhausted and need to be replaced.