Chlorine is very popular but the formulation of chlorine for spas versus pools is different. Don’t mix them up. Spa's have a different temperature range than pools. Use a DI-CHLOR product, not a TRI-CHLOR product used in pool chlorine. Spa chlorine is typically in dry granular form, while pool chlorine is liquid.
You can also use BROMINE, although some people have skin sensitivities to it. (I had a bad experience with bromine in high school chemistry) Also, it can cause discoloration of jewelry. More maintenance and monitoring than Chlorine.
Finally, there is a product called “Cleanwater Blue” which uses copper ions to kill bacteria and algae. It is concentrated and cost effective. It has none of the skin or jewelry problems mentioned above. You might want to check into it!
Good luck soaking!