JamesBenjamin,
I'm sorry to tell you this, but a foul-smelling tank is not a normal thing and I would look to the frequency/intensity of your tank maintenance as the cause, excluding the possibility that there may be a dead fish in there, that is.
Being in the medical field, I can tell you that a foul smell of any kind is linked to an over-growth of bacteria. Not to get into too much detail, but the foul smell itself is a by-product of the bacteria themselves. In the case of your aquarium, we could be talking about dead fish, overfeeding (i.e., rotting food), an accumulation of wastein the substrate, or dirty filter media.
First and foremost, make sure all your fish are accounted for. Then, not only do a major water change, but vacuum the daylights out of your substrate, taking care to do the same to your decorations. Check your filter media and make sure it does not have an over-accumulation of waste products. You may need to do these things once or twice more until you notice a change in the odor.
A foul-smelling odor is a symptom of something and while carbon can eliminate the "odor", it will not eliminate the "cause".
