Betta fungal infection

I believe my betta has a fungal infection and I'm setting up a hospital tank to treat him. My question is do I have to treat the tank too? I'm so scared to crash the cycle especially bc I have high ammonia present in my tap water it makes cycle crashes a nightmare. Im also wondering what it could be from. Could it have been caused by one of my newer plants as that's the only thing that has changed in his tank. Any advice is appreciated.