From what i have read you need a lot of carbon to be effective---one source i think said 1 cup per 25 gallons? it gets quickly exhausted but can become a surface for biological filtration.
I am currently trying purigen which is a resin made by seachem-check out there website,it's possible to regenerate and re-use under certain conditions.They recommend using in a canister as apposed to HOB power filter.Little fishes makes a reactor that is perfect for purigen,but requires a flow rate i think of about 90 gph.
The negative aspect of purigen is whether it can take out too much---this i don't think has been determined.Correct me if i am wrong but this is the problem with carbon being a source of hole in the head disease--taking out essential elements in the water required by fish.
Other than using carbon for removing odors--i use it just in my filters hooked up to my fluval surface skimmers for protein removal-i would personally not use it as there are alternate ways to polish water
