well, its my opinion that the carbon removes some particles and it clears up the water, like polishes it.
the way i use it is one, rinse it thouroughly, then put wherever u use it, i put it in my sump. i leave it sit for about 3-4 days. then i take it out, wash it out good, then let it dry out. I do not put it back in the tank. about 3-4 weeks later, i use the same carbon again, i place it in the tank for 3-4 days, then repeat the cycle. i ususally get about 2-4 uses out of each baggie i make up.
i dont leave it in the tank constantly because after all the carbon fills up with the bad stuff, then the bad stuff breeds in your cabon bag and starts to produce. atleast thats my opinion.
so for what thats worth, thats how i use it.
jb