How do you know it's truly fin rot?
If it is fin rot, you need to find what's causing it and address that problem. What is your water quality like? What are your pH, ammonia, nitrate, and nitrite readings? How often do you do water changes, and how much do you change each time?
If water changes fixed it the first time, I'd say it's an issue related to your water quality and you should probably increase the amount and/or frequency of the water changes.
If it is fin rot, you need to find what's causing it and address that problem. What is your water quality like? What are your pH, ammonia, nitrate, and nitrite readings? How often do you do water changes, and how much do you change each time?
If water changes fixed it the first time, I'd say it's an issue related to your water quality and you should probably increase the amount and/or frequency of the water changes.