Carbon is not needed full time in most tanks. It can be sued all the time, but it must be replaced often to remain productive. After a certain time period, it will have pulled out everything that it can, and ceases to function, though it will be colonized by beneficial bacteria. Is the carbon attached to the other filter media, or is it discrete? If it's possible to remove just the carbon, you can, and then decide if you want to replace it or not. If it is attached to the other filter media (ie, a sponge), then you will just want to rinse the cartridge off and return it to the tank; replacing the entire thing will remove the benefiial bacterai, causing the tank to re-cycle.