This specially formed filter material is for use after layer of eheim mech or mech pro. Eheim fix acts as a seperation layer between the mechanical and biological filter media. This is where the larger dirt particles are trapped thus removing them from the water before entry into the biologically functioning filter layers. To fit an eheim external filter we also offer eheim coarse filter pads to acheieve the same result. Reuseable suitable for both fresh and saltwater.