Well, I've never encountered FW carbon as opposed to SW carbon. I don't know if there is any difference in the two, whether one could be harmful or not, but can try to clarify the argument of whether activated carbon in general is detrimental. Carbon, UV Sterilizers, and most of the chemical means of filtration in our tanks are indiscriminate in their removal of certain compounds, etc, and thus will take the good with the bad. So, using Activated carbon 24/7, can possibly harm the reef, by ridding it of essential minerals and bacterias, along with those that are harmful. Some agree with this, some disagree, but this is the only reason I've ever encountered for not using carbon, as for the veracity of this claim, I'm unsure, as I know of many with great reefs that use carbon, and vice versa.