Pro edition
Chemistry Guide
The arithmetic, and the correction order most owners get wrong. All doses are per 10,000 gallons.
Correct in this order
pH is not first. That surprises people, and it is the reason a lot of pools never hold balance.
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Total alkalinity first
TA is pH’s shock absorber. Correcting pH while TA is wrong means pH drifts straight back, and you burn acid achieving nothing.
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Then pH
Once TA is in band, pH will hold where you put it.
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Then calcium hardness
Slow-moving and rarely urgent, but it decides whether the water eats plaster or coats it.
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Then cyanuric acid
Only ever adjusted upward by adding, and downward by dilution. Check it before you top up stabilised chlorine, not after.
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Chlorine last
It is the output, not the input. Chlorine dosed into unbalanced water is money poured into the skimmer.
Dosing, per 10,000 gallons
| Goal | Product | Dose | Note |
|---|---|---|---|
| Raise pH | Soda ash (sodium carbonate) | 6 oz per 10,000 gal raises pH ~0.2 | Pre-dissolve. Broadcasting it clouds the water for a day. |
| Lower pH | Muriatic acid | ~12 fl oz per 10,000 gal lowers pH ~0.2 | Add to water, never water to acid. Circulate an hour before retesting. |
| Raise alkalinity | Sodium bicarbonate | 1.5 lb per 10,000 gal raises TA ~10 ppm | Also nudges pH up slightly. Expect it. |
| Lower alkalinity | Muriatic acid, poured in a column | Iterative — 25 fl oz per 10,000 gal lowers TA ~10 ppm | Aerate between doses. This is a multi-day job, not a single visit. |
| Raise calcium | Calcium chloride | 1.25 lb per 10,000 gal raises CH ~10 ppm | Exothermic. Add slowly and never to a skimmer. |
| Raise CYA | Stabiliser (cyanuric acid) | 13 oz per 10,000 gal raises CYA ~10 ppm | Dissolves slowly through the skimmer sock over days. Do not retest for a week. |
Figures are standard industry approximations. Always add chemicals to water, never water to chemicals, and never mix products in a bucket.
The cyanuric acid trap
- CYA protects chlorine from UV — without it, an outdoor pool loses most of its chlorine in hours
- It also binds chlorine. The higher the CYA, the smaller the fraction of your free chlorine that is actually available to sanitise
- A common working rule is to hold free chlorine at roughly 7.5% of the CYA reading
- At 30 ppm CYA that means about 2 ppm chlorine. At 90 ppm it means nearly 7 ppm — which most owners never dose to
- So the pool reads "3 ppm chlorine, fine" and is chronically under-sanitised. This is the single most common cause of a green pool with normal-looking numbers
- Stabilised chlorine (dichlor, trichlor) adds CYA every time you use it, so CYA creeps upward all season without anyone adding stabiliser deliberately
Drain-and-refill maths
- How much do I drain?
- CYA only comes down by dilution, and it dilutes linearly. To halve it, replace half the water. To go from 90 ppm to 45 ppm, that is a 50% exchange.
- Is there a chemical that removes it?
- Nothing that works reliably at domestic scale. Products exist; results are inconsistent enough that we do not sell them.
- When is the right time?
- Spring, before demand climbs, and not in the hottest part of an Arizona summer when evaporation concentrates what is left.
- What about the calcium?
- In hard-water regions a partial drain fixes CYA and calcium at once, since both concentrate through evaporation. That is the honest silver lining.