Fin rot help?

I am a noob who was recently gifted a beta fish and now I’m doing everything I can to keep him alive. He made it through the night but is weak (he was in a cup at a pet atore for almost 2 months) and I suspect he may have fin rot. Any treatments I can try that won’t kill my plants?

I am a noob who was recently gifted a beta fish and now I’m doing everything I can to keep him alive. He made it through the night but is weak (he was in a cup at a pet atore for almost 2 months) and I suspect he may have fin rot. Any treatments I can try that won’t kill my plants?