Theres a lot of debate on whether carbon is necessary for freshwater aquariums. Some say the use of carbon may be linked to diseases such as HITH because it removes essential minerals fish need and that carbon, if left to the point of over exhaustion then it would release harmful chemicals. I don't use carbon in my tanks, but with a lot of reading, I came to the conclusion that carbon isn't harmful. Anyway...
IMO carbon should only be used for removing medications but if your tank has a funky smell, I wouldn't see any harm in using it.