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No chlorine reading with ozone system

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We have an ozone system fitted and we were told to add 2 tsp of chlorine per week and not worry about the chlorine reading. The chlorine always seems to be at zero (based on the white test strip colour) even a couple of hours after chlorinating.

Also I have constant issues trying to lower the PH which just goes right back up again, even though my Total Alkalinity is pretty stable, and the water sometimes becomes a bit cloudy, smelly or foamy, or even fizzy.

I don’t know if these issues are all related or separate? I am constantly adding chemicals and at my wits end! 

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Tub is 700 litres.
After filling and heating I add 175g Hardness Increase.
Then 175g PH Buffer/Alkalinity Increase.
Then test using AquaChek test strips.
My PH is generally high so I add 25-30g PH Decrease, usually whenever I check it approx twice per week.
Then 2 tsp granulated chlorine once per week (Sodium Dichlor Di-Hydrate 560g/kg).

Occasionally I take a sample for testing, the last readings/prescription were as follows:
PH 8.1 / add 20g PH Decrease
TA 87 / add 25g PH Buffer
CH 223
CL 0.09
CA 56
Phosphates 377

Since that time I have completed a partial water change (approx half) and topped up the Calcium Hardness and PH Buffer accordingly - I did this due to an unpleasant smell and discolouration of the water.
The water is now very fizzy with the jets on and some foam/scum is floating on top, so I'm thinking a full water change might be best. 

Any advice on initial set up and ongoing chemical balance would be very much appreciated thanks! 

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I was hoping @waterbearwould jump in here. He's the chemistry expert.

Go to the chemistry section of the forum, or use the search tool, and read some of the threads about dichlor/bleach, ozone, ph, alkalinity, and such. Then come back with any questions, I'll do my best to answer them.

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Ok I have done some reading and got suitably freaked out about biofilm, white mould, chlorine gas inhalation etc etc! I have also run pipe cleaner through the tub, drained it and refilled it again, it’s now up to temperature and ready to start with new chemistry.   I’m hopeful that we don’t need any more drastic decontamination procedures at this stage as we haven’t had any skin or health concerns and the tub is still fairly new (bought Oct 2020), with this being about our 3rd water change. 

So my questions now are:

1) How do I gauge our chlorine demand when I was told not to worry about the chlorine reading because of the ozone system, and to just put in 2tsp of dichlor per week and after high usage or cloudy water? Initially we tried using extra dichlor but our CYA levels went too high. Should I add bleach instead of dichlor for this reason? Or can a different form of chlorine or bromine be used to get around this issue?

2) What are borates used for?

3) I’m confused about the relationship between TA and PH. By raising my TA to 100-150ppm as recommended, am I also increasing my PH? I have constant issues with high PH readings above 8 and am constantly adding PH Decrease only to have it shoot right back up again the next day.


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1) You need to add chlorine after each use. I use 1tsp plus 1/2 tsp per person after use. I also use nature2. I would recommend more chlorine without the silver ion of nature2. If your cya reaches 30, switch to bleach or plan your next drain and fill in the near future. I drain mine every 3 months.

Yes, the ozone will get rid of excess chlorine as well as the combined chlorine from sanitation. This means no chemical odors or high chlorine level when you use it. It does not necessarily mean you use less chlorine, but can let you get away with using more.

2) PH stabilizer. I don't use it.

3) Alkalinity and PH are linked, and any change to one changes the other with the exception of aeration, which raises ph. Get your ph balanced and see where your alkalinity lands. In most cases you will be fine with that. 

Keep air controls closed when not in use to lessen ph rise from aeration. 

@waterbear, care to jump in here?


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Since ozone tends to destroy chlorine you actual chlorine usage might very well be higher with it than without it. You want to maintain a FC level of 1-3 ppm with NO CYA, 2-4 ppm with 20 ppm CYA, and 3-5 ppm with 30 ppm CYA. IF you are using a mineral sanitizer (Nature2 or similar) you can run a a very low chlorine system by using MPS after every use and shocking with chlorine weekly (MPS and Silver at high temperature IS a sanitizer) or a lower chlorine system (running about 2 ppm with CYA at around 30 ppm) but I do not recommend them for various reasons that are technical)

Borate is more than just a pH stabilize (and very useful in spas because of the amount of aeration which is the main cause of pH rise because of the outgassing of CO2)and I swear by them. Not only do they help stabilize pH, they have algaestatic proprieties nd also change the 'feel' of the water (more silky), and the "look" more 'sparkling)

Because of the amount of aeration in spas from the jets, bubblers, ozone injectors, blowers, etc. it is better to run a very low TA to minimize the pH rise. 50-70 is a good range. TA is a measure of the bicarbonate ions in the water and the higher the concentration the faster the pH will rise because of the aeration. The additon of 30-50 ppm borate helps offset the fast pH rise that can occur.

Read these (The second one explains why pH rises when TA is high and also is a howto on how to properly lower the TA):




Rule of thumb, get your TA in line and then adjust the pH. The biggest mistake most people make is running the TA too high and trying to run the pH too low. IF you add borate you want to run your pH at 7.7 to 7.8 and not lower it until you hit 8.0.


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BTW, alkalinity increaser is nothing more than baking soda! The ingredients will say either sodium bicarbonate or sodium hydrogen carbonate, both are the same thing, baking soda!

Also, with a Cyanuric acid (CA) level of 56 you need to maintain a FC level of 5-7 ppm because of the interaction of cyanuric acid and chlorine. You need to stop using dichlor and switch to liquid chlorine (pool chlorine or laundry bleach, the plain unscented unthickened kind. They are both Sodium Hypochlorite and will not add any additional cyanuric acid. Dichlor adds 9 ppm Cyanuric acid for every 10 ppm chlorine added and the cyanuric acid is cumulative since it is not consumed like chlorine.

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