The four duo-pack of Fluval Nitrite Remover provides incredibly effective chemical filtration for freshwater and saltwater environments. The foam sponge was designed to eliminate harmful nitrites and nitrates which contributes to a healthy aquatic environment and dramatically reduces the instances of fish loss. The foam sponge also traps floating particles such as debris, waste or food. The sponge dramatically increases filtration quality and efficiency. The Fluval Nitrite Remover can be replaced monthly or as required after water testing. Ideal for new, dirty or heavily stocked aquariums.