Carbon in a reef tank. One main reason you shouldn't have it in th etank is because it is simple removing most chemicals/non-biological things out of the water. For instance, when you pplace a new bag of carbon in the water and add the nutrients and chemicals to liven your coral up, the carbon will remove the additives. That is what it is made for to remove most suspended chemcials from the water, its like not adding anyhting at all. If you have had carbon in the tank right now its most likely dead, run out of juice your could say, bascially and now is just a home for beneficial bateria. I see no reason why u should take it out since it has already run its course. Carbono is best used for quarentine tanks and fish only tanks, because if u medicate or spill something unwanted into them the carbon will help greatly in the removal of that unwanted chemicals.