Sorry you are having such problems. Doing 85% changes is only diluting the ammonia unfortunately. In a 1G, you need to remove the fish and do 100% changes. This is probably why the fin rot won't go away.
So we know more about your fish, can you answer these...
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
In a 1G, you need to remove the fish and do 100% changes. This is probably why the fin rot won't go away.
Exactly. I'm fighting fin rot now and have had the best results with 100% daily water changes while Harleys in a 1g QT bowl. I have also added API Stress Coat to help promote new fin growth and have had some re-growth with that. The aquarium salt didn't work for me but it has for others. Good luck!