The carbon needs to be changed after a month anyway !!! It becomes ineffective after that.
EDIT: It does absorb trace elements that plants need. But it should not affect them too much (probably slow down their growth). If you have a heavily planted tank then it could make a difference.
You should look at the root cause of the smell....
A water change should get rid of the smell. If its a stale water kind of smell then its normal. Excess algae can cause a stronger smell. If its a bad smell like that of rotten eggs you are probably overfeeding.
And yes, using carbon should get rid of the smell (temporarily)
I agree; healthy tank has virtually no smell. There's something wrong there. If it smells like fish, then it's most likely that your fish are under stress. Other smell are often caused by decomposition under anaerobic condition.