Fluval Bio-Foam Filter Block. Fluval Foam captures large particles and debris for effective mechanical filtration, which is the crucial first stage in the filtration process. Custom fit to prevent debris bypass, the foam has a pore size that allows efficient water flow with less clogging for long-lasting filtration. The foam also enhances biological filtration. All have 2 pieces except FX4/FX5/FX6 foam has 3 pieces. Mechanical filter media Effectively traps majority of waste and debris Complex foam structure maintains consistent water flow and naturally promotes beneficial bacteria growth Custom fit to prevent debris bypass Enhances the biological filtration process Replace every 6 months For use in freshwater and saltwater aquariums.