Macropore is a macro-porous synthetic polymer that controls nitrates in freshwater and marine aquariums by filtering out nitrogenous waste products, resulting in crystal clear water. For freshwater aquariums, it can be recharged with a salt solution, while marine aquariums and occasional freshwater recharges require a bleach solution. The product is used in a mesh bag within the filter and needs to be replaced when it no longer clarifies water or bleaches white.