What size tank and what are the tank mates? Is the tank cycled? Do you know the ammonia, nitrite, nitrate levels? What is your water change schedule?
Fin rot is usually caused by unhealthy water, so I'd start there and make sure your water is healthy. How severe is the fin rot?
I'd start with a 50% water change with a good dechlorinator. If the fin rot isn't too bad and not infected daily water changes should help it heal on its own. If it's severe or/and shows signs of infection then perhaps an anti-bacterial med would help (you may want to QT the sick fish though rather than treat the whole tank).