First of all, take the sick fish out and put them in another tank if you can. Second, I don't like putting salt in the aquarium itself--just do what's called a salt bath, where you get a container, fill it with tank water, then put a little salt in there (not sure what the exact concentration is). My LFS sells salt for this purpose, but I'm not entirely certain that it's anything other than non-iodized salt... They claim there's something different. Anyways, iodine is bad for your fish.
Kirianna, the carbon is the little pouch of black gravelly looking stuff in your filter... What kind of fish, how big is the tank, when did you buy them, etc etc...