The AquaClear Zeo-Carb Filter Insert is a replacement filter insert that combines the power of both activated carbon and ammonia remover for a clean freshwater tank. It contains a natural ammonia-removing resin that forms a chemical bond with ammonia to remove it from the water in your aquarium. The premium research-grade carbon creates a large surface area that absorbs impurities and removes odors and discoloration for a clear aquarium. The AquaClear Zeo-Carb Filter Insert is an alternative chemical stage replacement that gives you the benefits of both filter inserts. With this dual-purpose insert, you can remove impurities along with toxic ammonia and nitrate for a healthy, stress-free environment for your fish.