The AquaFX Deionization (DI) Aquarium Filter produces lab-grade water to keep your aquatic pets healthy and happy. This filter uses nuclear-grade mixed bed， color exhaustion indicating deionization resin. It’s an ideal choice for the final polishing of your reverse osmosis water， for emergency fast water production and for tap water purifier replacements. The resin changes color from blue/black to red/orange as it exhausts， so you know just when you need to replace the filter to keep your pets healthy. The refillable shell makes for easy reuse.