Ofuro Water Additive

Greetings,

I lived in Japan for a while about 35 years ago, and I remember that there was an additive which I dissolved in my ofuro water to keep it fresh, so I wouldn’t have to change the water every day. I can’t remember what it was, and googling hasn’t been much help. Can anyone tell me some common additives they use today? I’m hoping to run across something more natural and less chemical.

Thanks in advance.