The tri-sulfa available here is in the photo below, but I assume the ingredients are similar.
If you decide to use it you’ll need to remove any carbon in the filter (if you have it in your filter), and note that it will affect your beneficial bacteria so you’ll have to watch out for that and keep up with water changes. You’ll know it because your ammonia level will creep up after around 2 days of treatment.
I have success treating white spot but not much else, and I don’t have fin rot so I won’t say how effective tri-sulfa is for fin rot. The problem is we don’t really know what bacteria infecting our fish, so we guess on the course of treatment, the success rates are not good.
I don’t know if pH is the main cause of this fin rot, but we can be sure that poor water quality and the general environment in the tank cause all sort of problems for the fish. pH, GH, ammonia, etc.
A good practice is to observe the fish and take note of any changes in fish behaviour. Large tank is way better than small one because the water quality is a lot more stable in large tank, although I understand that not everyone has room for a large tank.
All the best.