Carbon, when it has the capacity, absorbs chemicals and helps to "polish" the water. Only problem is 1) it doesnt discriminate when it comes to chems, and 2) it burns out fairly quickly. I wouldnt be worried about replacing your filter cartridge every 4-6 weeks as the big companies say. Simply rinse the cartridges of in old tank water and put them back in. Your tank will stabilize alot quicker