|Not necessarily. Activated carbon is sometimes regarded as detrimental to saltwater aquariums because it removes trace elements especially iodine from aquarium water. However, this is easily remedied with regular water changes and periodic use of trace element supplements.
Activated carbon offers important benefits for saltwater aquariums. It removes a wide variety of organics and chemicals from the water. In fact, activated carbon performs much like a protein skimmer, with the added benefit of water clarification. Many authors have even suggested that activated carbon, if used properly, can replace a protein skimmer in a saltwater aquarium.
To use activated carbon properly, place it in a media bag and incorporate it into the filtration in an area of high water flow (for example, within the sump near the water return line, or within the hang-on filter). Be sure to follow the manufacturer's recommendations for the quantity needed and replacement timeline.