Replacement Foam Filter Block intended for use with larger models of the Fluval Flex aquariums (57L and 123L). This Fluval Foam Pad captures large particles and debris for effective mechanical filtration, the crucial first stage in the filtration process. Custom-sized, it fits perfectly inside the filter chamber to prevent debris bypass. The foam pad has a pore size that allows efficient water flow, less clogging and long-lasting filtration. It is recommended to change your aquarium filter sponges once every few months or when it no longer holds its shape. It is best to not replace the foam and biomax at the same time in order to keep the bulk of your beneficial bacteria present for biological filtration.