Your betta doesn't have finrot, its tail biting. the egdes are very clean, you can read about tail biting here
you should really stop treating him, tail biting sometimes it triggered by stress and constant medication is definitely stressful. the best remedy for tail biting is extra water changes to keep this tank free of ammonia and other poisonous wastes and higher temps of 80f-82f.
p.s. there is no such thing as "betta" salt, antibiotics, water, etc. this is just a way a company tricks people into buying there stuff. chances are, any product with the word betta attached to it don't do anything.