Here's an interesting article about the use of carbon. http://www.thetropicaltank.co.uk/carbon.htm
I personally just got the AquaClear 20 for my tank when the old filter died. This filter has a alone for mechanical filtration, then a carbon bag for chemical filtration, and finally a bag of pellets for bio filtration. I didn't bother with the carbon I just added a second bag of the bio filter beads. I'll save the carbon for if I ever need chemical filtration.