Carbon can be useful if you have corals and want to keep your water polished. Removing the yellow pigmentation from the water helps with light penetration (raises PAR), so it can be very beneficial to SPS corals and other corals sitting lower in the tank. Most people don't notice it, but you can tell if it's there by draining some tank water into a clean white 5 gallon bucket during a water change. It also helps with DOC reduction. I run a couple of cups of ROX 0.8 in the second canister of my dual GFO reactor. I can tell a difference in water clarity since I set it up...