I have a betta so Melafix is a no.fjh said:Melafix comes to mind. I've only used it once before on an angelfish but it worked pretty well.
Not sure what fish you are treating, but if he/she has a labyrinth organ (bettas, gouramis, etc) then use something else, like maracyn 1 or 2. If it is really severe, you might start off with a salt bath as well.
In all honesty though, almost any fin rot can be cured with good water conditions. It is also a lot cheaper to do 50% daily WCs than to buy medication.