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Hot Tub & Hard Water

jameson_uk

Member
Joined
10 Jun 2016
Messages
871
Location
Birmingham
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
 
Last edited:

MichaelJ

Member
Joined
9 Feb 2021
Messages
1,710
Location
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.

Cheers,
Michael
 
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