[Aqua-holic]

Some people use carbon all the time and others never use it I think it is a matter of preference. As for when it should be changed is dependent upon the type of carbon you use, the better grade of carbon the longer it will last. standard filter cartridges are usually only good for about 1 month but the higher grade or medical grade carbon can last for 3 months or more. spent carbon will rerelease all the things it has removed back into the aquarium if it is left in too long.