My experience with fin rot is that it's 90% of the time due to ammonia or some other contaminant. Make sure your ammonia is 0.
Salt will likely harm plants at level high enough to assist in some forms of fin rot. I've never had it do a lot of good.
If your ammonia is 0, you might try Melafix or Primafix, I can't remember which is for what. Should be plant safe.