I think my betta has a sore!

[Leopardfire]

My male betta has very severe fin rot and he is currently in a 1.5 gallon QT tank. I am using bottled water with minerals added and 1tsp per gallon of salt with 100% daily water changes to treat him.
This is his 2nd day of treatment and when I looked at him today, I noticed a darkish patch on his body by the base of his tail. It looked like the scales might be gone or it could be a sore, but the light is bad so I can't see it very clearly. His fins still have about a cm or more left, but do you think this could be body rot? If not, what do you think it might be?
Should I start using meds instead of salt? If so, what meds could I use? I have Triple Sulfa at home, but I could go get whatever he needs.

[Sena Hansler]

Need a picture. It is very possible it could be columnaris, or something just as strong. Why are you using bottled water? Is there something wrong with your tap water? Have you been just adding the salt, or pre-dissolving?

[MoonShadow]

A picture would be great! It's hard to help without seeing what you are talking about. It could be a rub, a sore, the start of colmunaris or other bacterial infections, but it's hard to give you advice without seeing it!