No matter what sanitizer you use, you will still have to balance your water. That means you need to adjust the pH and TA. Don't think of this as "adding Chemicals," rather, it is neutralizing the chemistry of the water as it comes out of the tap. If you keep your pH around 7.4 and your TA around 100, you can drain your tub anywhere: it will be better than the water which comes out of your tap and/or sprinklers.
I recommend Chlorine. If you do it right, you will not have chlorine odor, dry skin etc. How does that work?
If you add a small amount of Chlorine as you exit the spa, it will do it's job and go away by the next time you come to use the spa. If you have a very slight amount of chlorine your water is safe and clean, and should not smell of chlorine.
It is much easier to keep the tub clean, safe and clear if you have a good Ozone system and use Nature2.
Most of my customers who have voiced concerns about chlorine have been pleased to find this system works.