API nexx Nitra-Zorb - Size 4 - 2 Pack API nexx Nitra-Zorb is a combination of natural and synthetic ion-exchange resins that selectively remove ammonia, nitrite, and nitrate from freshwater aquariums. It’s also great for starting a new aquarium, as it prevents unhealthy ammonia surges. Completely phosphate free and can be reused after recharging.