Look up fin rot on google images and make sure that is what you are seeing-
If you are positive that it's fin rot, what you would want to do is use Aquarium Salt, Epsom is used for internal problems such as constipation, bloating, etc.
To treat with AQ salt, you are wanting to do 1 teaspoon of AQ salt (dissolve it before adding your guy in it), per gallon of water.. you will want to do daily 100% water changes, re-adding in the AQ salt each time (don't forget the water conditioner). You will continue that treatment for no longer then 10 days.
You should start to see it clear up in that 10 days.. if it hasn't gotten cleaner by the 6th day, you can up it to 2 teaspoons per gallon.