Ok heres how you would do a salt treatment.
Aquarium salt is perfectly fine for treating diseases, it's long-term use of it that can be harmful.
To treat fin rot, keep the water about 77-78 degrees F, and add 1 tsp per gallon of aquarium salt (condition/dechlorinate the water as usual), and do daily 100% water changes. It's definitely easiest to put the fish in a small quarantine container if you have one that you could float in your main tank. Keep up this treatment for 7-10 days. If you have any Indian almond leaf or oak leaves, the tannins are extremely beneficial.
LolaQuigs wrote this not me, so all credit goes to her.