Fluval Foams capture large particles and debris for effective mechanical filtration, the crucial first stage in the filtration process. Custom fit to prevent bypass, they have a pore size that allows efficient water flow with less clogging for long-lasting filtration. These inserts also enhance biological filtration.  For freshwater and marine environments.  For Fluval filter models: 204, 205, 206, 304, 305, 306.