The four duo-pack of Fluval Phosphate Remover provides incredibly effective chemical filtration for freshwater and saltwater environments. The foam sponge was designed to trap floating particles such as debris, waste or food, as well as removing and controlling phosphates and prevents outbreaks of harmful chemical spikes. The custom fit of the sponge dramatically increases filtration quality and efficiency. The Fluval Phosphate Remover can be replaced monthly or as required after water testing. Does not impact pH or water hardness.