Melafix, along with very clean water, works very well for fin rot, as well as torn or nipped fins. Just do a water change before you add it, and then follow the instructions- I believe you need to dose daily for 5 or 7 days (I don't have a bottle handy to tell you for sure), and then follow up with another water change. Remove the carbon from you filter during treatment, if you use any. It won't affect the bio filter.