Fluval FX5 Aquarium Filter Foam Blocks 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. Fluval FX5 Aquarium Filter Foam Blocks are built for the Fluval FX5 Filter and work to ensure your aquarium stays clear and clean.