Carbon is really just a 'water polisher.' It won't (shouldn't at least) affect "water quality" (I'm assuming you're talking about testing stuff here? (ammonia, nitrates, nitrites, phosphates, pH, etc). It will turn slightly yellow-colored water clear, as well as removes those impurities as mentioned (why carbon is so popular in the drinking water game).
Indeed, if used for to long, it will leech the contaiments back into the water. And if you take a look at the Disease forum, continued heavy use of carbon has been traced to HLLE in tangs and angels. Lastly for carbon to be any bit effective, water has to be forced through it. Simply dropping a media bag of carbon into your sump won't do squat, it has to be run through a reactor of some type, a common Phosban-reactor or something as simple as a powerhead's output attached to some PVC with carbon inside.
