Eheim 2010 Cartridge - Carbon 2pk  For adsorptive filtration     that is for the removal of harmful chemical substances (chlorine, residues of medications etc.)     there is a specially treated carbon cartridge. Adsorptive filtration is normally only necessary at the start-up stage of the aquariums or after treatment with medications. The filter cartridges should in any case only be used for a limited period (ca. 4 weeks) and then removed or replaced.  Key Benefits:  Specially treated carbon cartridges Removal of harmful chemical substances (chlorine, residues of medications etc.) Use only for a limited period (ca. 4 weeks)