The carbon would be to combat allelopathy between the corals. From Borneman's Aquarium Corals, "Sinularia flexibilis is a particularly toxic species to other reef inhabitants, with strong terpenes called flexibilide and dihydro-flexibilide... being acutely detrimental to (at least) some species of Acropora... In fact, many Sinularia species are classified as being in the 'most toxic' category... Anecdotally, it seems that this is even more pronounced in aquariums, where repeated reports of stony coral demise... occur regularly in the presence of multiple, large, nearby, or even isolated Sinularia specimens."
Just a thought.