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Hot Tub & Hard Water


10 Jun 2016
Not sure if there are any threads on this already as you can't search for hot tub....

We have just ordered a relatively cheap inflatable hot tub and I have had a quick scan of the things you are meant to do to keep it clean and healthy.
Everything I have read talks about the importance of getting the levels right and whilst I can apply some aquarium knowledge here, the chlorine bit is getting me as we spend time ensuring this gets no where near a fish tank.

The instructions state that the total alkalinity must be 80-120ppm (~5/6 dKH?) or the chlorine wont work properly and all sort of other bad things will happen.
Now my water is hard but is a bit odd in that the KH is only 5 whilst the GH is 12,. (Although I want to test this again soon as the TDS meter gave me a different reading to normal the last couple of weeks but I haven't had chance to test).

Is there any chemistry I should be aware of here? Is the reason for that alkalinity level to ensure the chlorine remains active? As an acid I guess the chlorine is trying to pull the pH down so a higher KH would be fighting this and lead to higher chlorine levels which aren't great for our skin? Does GH matter at all? I guess this will just lead to a build up of carbon deposits on everything rather than having any impact on the chlorine's ability to sterilise everything?

There is also the pH +/- chemicals. AIUI the minus is Sodium Bisulphate (Sodium Hydrogen Sulfate) which I am lead to believe will drop the pH without touching the KH? The plus is Sodium Carbonate(?) which I think raised KH, GH and pH? Assuming that is the case what could be used that will actually drop the KH? Partly thinking that if I do need to alter the water whether I can just by some chemical rather than paying a big premium to have it inside a hot tub manufacturer's branded container
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9 Feb 2021
Minnesota, USA
Hi @jameson_uk, The only thing we care about with our hot tub is pH (trying to keep it in the 7.2-7.8 range - which is usually stays at). We fill it will water from an outside tap that by-passes our water softener, so I figure the GH is ~20 and KH ~15. Otherwise we add a bit of "stain and scale" (to avoid calcium and other buildup) and shock (with some chlorine) almost weekly. The hot tub is equipped with a pretty efficient ozonator (last for 18 months) which allows us to go a pretty gentle on the chemicals - it's pretty nasty stuff... but the tub is always clean and fresh - we change water 3 x year... been running smoothly for over 5 years.