Originally Posted by gettankedaquariums

The rule of thumb for replacing carbon is every 4 weeks like clock work.

Here is a way to determine when to replace your carbon.

When you first install a fresh batch of carbon you will notice that you do not have to clean the interior of your aquariums glass as much, with-in a few weeks and you notice that you are cleaning you interior aquarium class more often means that your carbon is getting exhausted.

Good luck!