i don't use carbon. note that there are many types of carbon sold for aquarium use but VERY FEW of these are actually capable of adsorption in water. only HIGH QUALITY ACTIVATED GRANULAR carbon which has been degassed in an oxygen oven at temperatures close to 2,000 degrees Fahrenheit is functional --- anything else is worthless. i can recommend Marineland activated granular carbon in this regard though there may be others. degassing enlarges the apparent surface area of each carbon granule, thus greatly increasing the adsorptive capacity.
a great many carbon grades are washed with phosphoric acid in an effort to support the adsorption sites since this is cheaper than degassing. this phosphate will leach into your aquarium water and can reult in a major algae problem.
deep beds of Granular Activated Carbon (GAC) are always more effective than shallow ones of equal area. also, for a given amount of GAC the smaller the granules, the greater the adsorptive capacity. generally, there needs to be 5 to 10 grams of GAC for every gallon of water depending on stocking density. such carbon beds typically last only a month at best and more typically 2-3 weeks since most hobbyists tanks are overstocked. when the surfaces of the carbon become saturated, it stops working and needs to be replaced with new carbon.
if your filter uses prepackaged cartridges containing carbon, realize that these cartridges typically contain poor quality carbon and if quality carbon is used, only 20 percent or less of the recommended amounts of GAC is present. you can slit open the cartridge and fill them with more GAC but many pad filter types simply will not hold that much. thus, the carbon in the vast majority of filter pads on the market simply is not functioning.
in a filter, the GAC should always be placed after the mechanical filtering medium otherwise, the particulates in the water will quickly coat the surfaces of the GAC, rendering it useless.
so what's the bottom line? if you're going to use activated carbon, you've got to use QUALITY material, you've got use ENOUGH of it to work and you've got to CHANGE IT often enough to keep it functional. a weekly water change schedule taylored to your bioload will work equally well however and is certainly less expensive.
