I posted a similar question here a few weeks ago, and I have learned so much about hot tubs.
You have to treat the water. You have to keep water in the tub. Once it has been filled, the pipes wll never fully drain, and the acryiic will dry out and get cracks unless you keep water in it. So filling it up for a soak and then empying it is not wise. Plus 350-550 gallons of water for one soak is a tad wasteful, eh?
Most people with spas change the water 2-3 times a year!
I also wanted a chemical free spa. I don't want chlorine or bromine. I found an enzyme system on the web, which is 100% natural, but it did nothing, and I wouldn't recomend it if you find it. My spa got really slimy from it actually.
What we ended up with is the nature 2 system. It is a filter that slips into your exsisting filter, and it works great!!! It is the most natural thing I could find, and I obsessed on this for weeks. You do have to add monopersulfate, which is basically a dried form of salt and hydrogen peroxide. It is a chemical, but it isn't chlorine.
I also have an ozonator, which is pricey, but is also chemical free.http://www.nature2.com/spas.asp