Fluval BioMax Replacement Media for U2, U3, and U4 Series Underwater Filters provides an optimal environment for biological filtration. Each ring has a complex pore system that allows beneficial bacteria to thrive, helping to reduce and control ammonia and nitrate. Provides optimum biological balance Complex pore system supports colonization and growth of beneficial bacteria Reduces and controls ammonia and nitrate Replacement bio-media for Fluval U2, U3 and U4 Underwater Filters Directions: Rinse before using. Safe for freshwater, marine, or reptile habitats.