I run it in my fluval but just as ernie says a few days a month if the water looks less than pure.
My theory is that the Live rock has all this bacteria and all these critters microscopic and visible that filter the water, If I run a filter than the Bacteria and the critters wont have to do there job and will therefore not be as abundant. I assumed that if I used the filter less than I would get more of these critters and less mechanical filtration would be needed to the point of a somewhat self sustaining ecosystem
If you dont run live rock and live sand... then sure carbon would be great.