I don't use melefix or bettafix as I've been advised by breeders not to since they can harm the fish. When our combtail had fin rot I upped his temperature to about 80 degrees F and changed his water more frequently to keep it clean. He got over it and regrew his fins beautifully.
Edit:
Also, I have found this for treating finrot as well.
"Treatment: Conservative: Treat with Aq.Salt at 1 tsp/gal. Increase water changes to 100% daily. Replace accurate amount of salt following water changes. Add Stress Coat to help repair tissue. If there is little to no improvement within the first 5 days, you can increase the salt dosage gradually to 2tsp/gal but do not continue any salt treatments past 10 days. Medication: If Conservative treatment is ineffective use API Tetracycline, API Fungus Cure, API Triple Sulfa, OR API Erythromycin. Also add Stress Coat to help regrowth. Continue until fins/tail stop receding and start showing some new growth."