I run carbon 24/7 for several reasons. One is that the water develops a yellowish tint within a couple of weeks if you don't. This can actually cut down the ammount of light penetrating the water. I know of one guy who "burned" his corals accidentally when he started using carbon because it made that much of a difference in water clarity and the ammount of light reaching the corals. Two is that corals emmit a lot of very nasty chemicals when in competition(they don't just sting each other). We are, by jamming all kinds of different corals into a small space, doing everything possible to ENCOURAGE this competition to occur. And, unlike in the ocean where you have nearly limitless water volume to dissipate the toxins outside of an immediate area, our tanks can become saturated with them very easily. Carbon will adsorb most of this so I think it's prudent to use cabon quite a bit.