Hi,
Nordic water here 
Sweden to be precise.
I built a 1800L hot tub/spa/jacuzzi for outdoor use.
Technical was not to complicated.
After two year of perfection it was time to start using it, (admitting solving technical challenge is my work)
This was the time I understand how little I know about water!
First year was a learning curve.
Then I decided to keep maintenance and chemicals to minimum.
To make it more spontaneous friendly I added 40kw heater (wood) this way I don’t need to keep the water at 38C, (perfect for bacteria growth)
Now I can make it ready for swim in 2 hours.
This is made from Aluminum to make the heat transfer as high as possible. (This also make it sensitive for high salt and chlorine.
So I use UV and drive the pump 40min every 3 hours 24/7. Adding little hydrogen peroxide 18% every 2 week.
Also built a magnetic water treatment to keep the hydrogen more aggressive.
This year I have sent water sample to 3 party to make sure it safe. It’s fine!
Now I’m in process of installing ozone generator and skip peroxide.
The spa is made from glass fiber and rubber hose so it tends to raise the PH 0,4 every week, probably static friction.
All I need today is PH down every second week.
And small amount of peroxide.
The pump “change” total volume in 30min.
-Sorry for long introduction-