City tap water is kept in a huge tank the water is about 60 degrees it is treated and monitored 24-7, it is millions of gallons in a for the most part closed loop system. It is not 101-104 degrees open on the top to environmental items like bugs and pollen getting in the water and more importantly it doesn’t have up to six people sitting in it giving off sweat and body oils or people bringing in soap on their swimwear and all kinds of personal body products that get rinsed off in the hot water.
I started off with very lofty goals hoping the minerals and ozone would allow me to keep chlorine super low. I was using a frog inline dispensing system that I had hoped would control everything very precisely. We mostly have two bathers in a 400 gallon tub we shower every time before we use the tub and don’t use any swimwear.
Doing everything right to the best of my abilities I found it very hard to keep both my levels super low and the tub clean. When you have a process working at the lower limits like that all it takes is the slightest miscalculation on bather load and then a day of inactivity and you will open the tub to find it skunked. Of course this is just my opinion based around owning a tub for 3 years now.
I can’t give a percentage of how much the other things built into a tub reduce sanitation demands as I’m sure something like an ozone generator does help when the unit is brand new. Most people assume if they are making bubbles they are making ozone and that is not the case. I personally don’t think the minerals do much if anything.
Chlorine or bromine are your sanitizers if you look at the level used in public pools and tubs you would faint, but then again compared to how much Clorox people put in the laundry public pools is just a trace amount.
The method I found that works best for us is dosing the tub right after evening usage and then allowing it to come down over night. If done right the next day you will see the low level you desire. If usage was really heavy say 4 people with swimwear and more time in the water, you will get used to knowing how much additional to add.
Just my 2 cents on the subject. Be prepared for a learning curve.