Help keep your underwater companion’s water column crisp and clean EHEIM 2211 Canister Carbon Filter Pads! Designed to polish up your pal’s aquarium， they will keep water free from debris， odors and discoloration. They are engineered to help prevent clogging， as the last stage of filtering material in your EHEIM Canister Filter (sold separately). These carbon impregnated filtering sponges are ideal for restabilizing your ecosystem once you are finished medicating your aquarium. In order to prevent absorption of medication， they should not be used until treatment has been completed—and should be replaced every three to four weeks for optimal results.