Eheim Carbon Filter Pads are a great way to polish your aquarium water of debris, odors and discoloration. They should be replaced every 3-4 weeks for optimum results. This carbon impregnated filtering sponge material should be positioned as the last stage of filtering material in your Eheim Canister Filter to prevent rapid clogging. DO NOT use an Eheim Carbon Filter Pad when medicating your aquarium, it will absorb medication right out of the water. Use it immediately after you are finished treating with medications to help restabilize your ecosystem.