Don't over salt your tank.
How big is your tank that you're doing 10% water changes every day? That's pretty excessive.
Remember that as water evaporates the salt stays behind also, so always match your dosage of salt with the amount of water you're adding to the tank at that momment in time.
If it's fin rot for sure then you're adding to his stress by changing the water daily. What type of filtration are you using? It is possible you're doing more damage than good on the biological filter's level. Not allowing the tank to fully complete it's cycle. How new is this tank also?
I would do water tests currently before anything else, see what your chemical levels look like.