I use carbon in my reef tank three days before I do a PWC
. I guess in a perfect world you would not need carbon, but this is not a perfect world and carbon removes impurities that can build up in a reef setting. IMO
removes gelbstoff (the compounds that give water in an aquarium the yellow tint), some large organic molecules, medications, chlorine, pollutants and toxins, as well as many other types of chemical elements and compounds from the water that a protein skimmer or another means of filtration may not remove."