Check the listing for details. API Rena Filstar Nitra Zorb Pouch (old 726A) Size 6 Fits All API Filstar Filters. Condition: New. Listed at 15.68 USD. The Rena Filstar Nitra-Zorb cartridge is the first water filtration medium that removes ammonia, nitrite and nitrate in freshwater aquariums. Nitra-Zorb is a blend of natural and synthetic ion-exchange resins that selectively removes ammonia, nitrite and nitrate. Nitra-Zorb, when used in new aquariums, controls the level of toxic ammonia and nitrite during the initial development of the biological filter. In established aquariums, Nitra-Zorb helps control sudden spikes in ammonia and nitrite from the addition of new fish or anytime ammonia or nitrite is detected. If you have trouble controlling ammonia, nitrate or nitrite levels in your aquarium adding NitraZorb can improve water quality dramatically in a very short time.