Cyanuric Acid: Stabilizer, and Its Limit

What it does, and why every outdoor pool needs some

Ultraviolet light destroys free chlorine quickly. HTH's chlorine stabilizer page states that cyanuric acid prevents up to 90% of chlorine loss from sunlight, which is the difference between a pool that holds a residual through a summer day and one that does not.

Cyanuric acid binds loosely to chlorine, holding a reserve that releases as free chlorine is consumed. It is genuinely essential outdoors, and it is why an unstabilized outdoor pool is an expensive way to make chlorine disappear.

The targets, and where they come from

The limit, which is the part nobody mentions

The same binding that protects chlorine from sunlight also holds it back from doing its job. Past roughly 50 ppm, an increasing share of your measured free chlorine is bound rather than available, so the number on your test overstates what the chlorine is actually doing.

This is the single most common explanation for the most confusing situation in pool care: a green pool with a chlorine reading that looks fine. The chlorine is there. It is just not working.

It only leaves by dilution

Cyanuric acid is not consumed, does not evaporate, is not filtered out and is not broken down by chlorine. It accumulates, and the only way to reduce it is to remove some of the water and replace it.

How pools get there

• **Trichlor tablets, every week, all season.** The commonest route by far. Each tablet adds stabilizer as it dissolves.
• **Dichlor shock, used routinely.** Roughly 0.6 ppm of CYA rides along with every 1 ppm of chlorine added.
• **Deliberate over-dosing at opening**, usually because stabilizer dissolves slowly, the first retest reads low, and a second dose gets added before the first has finished.
• **Topping up rather than diluting.** A pool that only ever loses water to evaporation concentrates everything dissolved in it, including CYA.

Staying out of trouble

1. **Test cyanuric acid a few times a season**, not once a year. It only moves in one direction.
2. **Under-dose when correcting it.** Too little is easy to fix; too much needs a drain.
3. **Wait 24 to 48 hours before retesting** after adding it, since it dissolves slowly and an early reading tempts an unnecessary second dose.
4. **If you sanitize with tablets, shock with cal-hypo or liquid chlorine**, both of which add none. See cal-hypo vs dichlor vs trichlor.
5. **If CYA is climbing every season, change the sanitizer** rather than fighting it. Liquid chlorine and salt systems add none at all.

The picks

Top pick - Stabilizing a liquid-chlorine or salt pool

In The Swim Pool Stabilizer, 100% Cyanuric Acid, 25 lb

100% cyanuric acid at a purity that lets you compute the dose, for pools whose chlorine source does not supply its own.

The published figure

100% cyanuric acid, granular

Trichlor tablets and dichlor shock both add cyanuric acid as they dissolve. Liquid chlorine, cal-hypo and salt systems do not, so those pools need it added deliberately.

At 100% purity, 1 lb raises 10,000 gallons by 12 ppm. HTH's own stabilizer page states the useful targets: 30-50 ppm for trichlor and dichlor pools, 60-80 ppm for saltwater.

Cyanuric acid does not leave except by dilution. Adding too much is a mistake you fix by draining water, so under-dose and retest.

What it does well

• 100% purity means the arithmetic is exact
• Protects chlorine from UV loss, which HTH puts at up to 90%
• One correction lasts until you dilute the water

What it costs you

• Dissolves very slowly — 24 to 48 hours before a retest means anything
• Cannot be removed except by draining and refilling

Skip it if you sanitize with trichlor tablets. Those already add stabilizer, and your problem is more likely too much of it.

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Pick 2 - Saltwater pools

Clorox Pool&Spa Swimming Pool Chlorine Stabilizer

Saltwater pools burn through chlorine in sunlight faster than anything, and this is the correction, in a size that suits one.

The published figure

Cyanuric acid stabilizer, stated as saltwater compatible

A salt chlorinator generates chlorine continuously and adds no stabilizer, so a salt pool without cyanuric acid loses most of what it makes to UV.

HTH's stabilizer page puts the saltwater target at 60-80 ppm, higher than the 30-50 ppm for tablet pools, because the generation rate is steady rather than dosed.

Add it through the skimmer with the pump running and give it two days before retesting.

What it does well

• Explicitly stated as saltwater compatible
• Directly reduces the chlorine your generator has to make

What it costs you

• Very slow to dissolve
• Cannot be reduced except by draining

Skip it if you already read above 80 ppm — more will start suppressing the chlorine you generate.

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Pick 3 - Knowing the number rather than the color

Taylor Technologies K-2006-SALT FAS-DPD 9-in-1 Test Kit

FAS-DPD is a drop-count titration, so the chlorine reading is a number you counted rather than a color you matched.

The published figure

FAS-DPD titration; tests free and combined chlorine, salt, pH, alkalinity, hardness, cyanuric acid

Every strip and every DPD color-match kit asks you to judge a shade. FAS-DPD asks you to count drops until a color disappears, which removes the judgment entirely.

That matters most exactly where the cheap methods fail: above about 5 ppm free chlorine, color-match scales saturate and read 'high' for anything from 6 ppm to 30 ppm. When you are shocking, that is the range you are in.

It also measures combined chlorine separately, which is the reading that tells you whether the shock worked or the pool needs another round.

What it does well

• Titration rather than color matching, so chlorine is a counted number
• Reads accurately at shock-level chlorine where strips saturate
• Measures combined chlorine, cyanuric acid and salt in one kit

What it costs you

• Slowest kit here to use — several minutes per full test
• Reagents expire and need replacing each season
• The most expensive way to test

Skip it if you will not actually run it. A kit used monthly is worse than strips used weekly.

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