Many aquarists also use a protein skimmer to remove dissolved organics before they are broken down by the biological filter. This helps reduce the amount of nitrates that accumulate in the tank water. The live rock has so much surface area that it can provide all of the needed biological filtration. Both have microscopic pores that capture organic and other molecules and remove them from the water. The carbon is depleted when the pores are filled with organic molecules that are removed from the water.
Activated carbon is very effective at removing the organic dyes that discolor aquarium water.
A good marine aquarist makes good use of both mechanical and chemical filtration and changes the media in these filters at regular intervals. Some aquarists don't use carbon all the time because it may remove important trace elements, so they run carbon perhaps one week per month, just before doing a water change. A simple external filter that hangs outside on the end or back of the aquarium does an excellent job of mechanical and chemical filtration on a small marine aquarium. Although a lightly loaded marine aquarium can be run successfully without an auxiliary filter, such a filter greatly improves water quality and clarity. Activated carbon lasts for some months before it must be changed.