Nitra Zorb is a convenient pouch of natural and synthetic ion-exchange resins that selectively remove ammonia, nitrite, and nitrate from freshwater aquariums. Nitra-Zorb also helps during start up by reducing unhealthy ammonia surges while establishing the aquarium’s ecosystem. Nitra-Zorb is completely phosphate-free and can be reused after recharging. Contains 1 large pouch for up to 55 gallons aquariums. Fits most filters. Replace every 2 months. Shipping weight: 3/4 lb.