What does it do exactly?
Activated Carbon (AC) is a charcoal substance which is used in the aquarium to removes odors, removes color, and makes aquarium water as clear as ice. Carbon helps remove some smelly organic gases like hydrogen sulphide from the water, absorbs chlorine from water, or if your tap water contains chloramine, it removes the chlorine and leaves ammonia behind. Also small amounts of chelated copper, mercury and iron may be removed and this may be of limited use.
Carbon itself is one of the keys in the battle against unruly dissolved organic compounds and the algae fertilizing nutrients. Carbon, specifically activated carbon, has a chemical love for organics, acting like a molecular magnet pulling contaminants into its chemical embrace. Together with an effective protien skimmer, activated carbon plays a key role in keeping your aquarium healthy and looking good.