It is basically a common myth, Folks with chronic chemical problems, and less than correct analytical skills saw levels increase in their tanks after the initial drop created by adding carbon. from this they assumed (never good with science) that the same chemicals they removed from the water had leached back out into the water. When in fact they really had some other source adding the chemicals and the carbon reached it's limitations too quickly. In addition, there are plenty of folks in the buisiness who want to sell you as much carbon as they can so any way they can get you to remove it and replace it they will.
As a general rule, carbon is very limited in it's effective time frame, and largely unneeded in a well maintained tank. It can serve as a good bio-media, but not any better than most pourous rocks might do. I run none in my tanks, but keep it on hand for any type of chemical emergency that may arise. Since I don't use meds typically, the only time I might need it is when I paint near the tanks, or my Wife sprays something (doesn't happen often, she understands the ramifications). I have had the same container of carbon for many many years and the box is still sealed.
Dave
