^^^they don't...this is another aquarium myth that refuses to go away. Firstly carbon adsorbs, not absorbs. Carbon is able to let go of it contaminants, but it will only do this at extreme times in water. In an aquarium it takes a massive Ph swing for the carbon to release, the swing is between 3.0-8.4 and if that happened in your tank your fish will be dead along time before the contaminants affects the fish.