How do I treat my betta's fin rot?

alright so i have a male betta in a 10 gallon fish tank with neon tetras. The aquarium is heated and filtered but my betta has fin rot. My betta got fin rot when he was in a small bowl so i upgraded to a 10 gal. I treated him with Maracyn 2 and the tail was starting to grow back. I put him in the tank and the fin rot reappeared. I know the neons aren't nipping his fins because they stay away from him all the time. My question is if i can treat the whole tank with Maracyn 2. Will it hurt the neons? Should i put him back in the 2 gallon bowl while I treat him?