Specifically designed to fit inside the Fluval Nano Filter, the Fluval Bio-Foam captures debris for effective mechanical filtration, plus it has large surface areas that encourage the proliferation of friendly nitrifying bacteria, which provide efficient biological filtration. Custom-sized, it fits perfectly inside the filter chamber to prevent debris bypass. Bio-Foam has a pore size that allows efficient water flow, less clogging and long-lasting filtration. It is suitable for freshwater environments. Includes a total of 2 bio-foams (1 package, 2ct per package).