Betta fish, fin rot, illness?

I've had my new betta fish for about three months now, his tank is a 5 gallon and it has soft gravel, ALL silk plants, heater, filter, and a floating log (no rough edges on it). And he seems to be ripping/decaying his fins somehow,,, I always do water changes about every 2-3 weeks and i put water conditioner for Betta's and aquarium salt...i researched and thought that it might have been fin rot. but, fin rot has some brown spots on it...(usually) so i'm wondering what might it be? Should i buy him medication? Which kind? thank you!