Activated Carbon is an essential filter media for every aquarium. It can effectively eliminate odors, purifies and softens water through the process of adsorption (not absorption). To adsorb means to” take up” onto a surface as opposed to absorb which is to “take in”. Activated carbon therefore will extract harmful substances from the water and attach or retain them. These substances are, for instance, chlorine in mains water, heavy metals, residues of water treatments or traces of cleaning agents, pesticides, colourants etc.