The AquaClear Foam Filter Insert is a replacement filter insert that traps particles and debris to help maintain a clean freshwater or saltwater tank. The foam filter insert fits perfectly into the filter to prevent bypass. It also helps prevent particulate waste from clogging other media in the filter. It’s designed as part of the AquaClear system that provides three types of filtration: mechanical， chemical and biological. Although the AquaClear Foam Filter Insert provides mostly mechanical filtration， it also provides a type of biological filtration as well. The porous surface allows friendly bacteria to grow， which is essential in the removal of toxic ammonia and nitrate.