What PoolBoss Says
Raise cyanuric acid by adding about 13 oz of granular stabilizer per 10,000 gallons for every 10 ppm you need, aiming for 30-50 ppm on an outdoor chlorine pool. Put it in a skimmer sock or a bucket of warm water with the pump running, don't backwash for a couple of days, and retest after a week.
Almost everything written about stabilizer is about having too much of it. On a service route the opposite problem is common enough to cost real money: a pool that was drained for an acid wash, refilled after a replaster, or run on liquid chlorine for three seasons with no stabilizer going back in. The chlorine you add on Monday reads fine as you leave and is close to zero by Wednesday, and the customer calls about cloudy water before your next visit.
When cyanuric acid in a pool is low, the fix is cheap and the dose math fits on a sticky note. What catches techs out is everything around the dose: pool stabilizer dissolves slowly enough that a retest two days later reads almost unchanged, which is how a pool ends up with a second dose it did not need. Below is the dose and the target band, what each reading means for your chlorine, how to add conditioner so it goes into solution, how to tell a stabilizer problem from a chlorine demand problem, and the pools that should not get any at all.
At a glance
Key takeaways
- Add about 13 oz of granular stabilizer per 10,000 gallons for each 10 ppm of lift, and round the dose down.
- Target 30-50 ppm on an outdoor chlorine pool and 60-80 ppm on a salt pool.
- Dissolve it in a skimmer sock or a bucket of warm water with the pump running, and skip backwashing for 48-72 hours.
- Retest at the next weekly visit, not two days later, because stabilizer can take 3-7 days to fully dissolve.
- If chlorine holds overnight but disappears on sunny days, suspect low stabilizer before you suspect algae.
- Treat a reading below your kit's lowest mark as unknown, and dose toward 40 ppm rather than toward 50.
- Never add stabilizer to indoor pools or bromine pools, and check the state code before adding it to a commercial pool.
How do I raise cyanuric acid in a pool?
Add granular cyanuric acid, sold as pool stabilizer or pool conditioner, at about 13 oz per 10,000 gallons for every 10 ppm you want to gain. That comes from simple arithmetic: 1 ppm in 10,000 gallons is 0.0834 lb of a 100% product, so 10 ppm is 0.83 lb and 30 ppm is about 2.5 lb. The two retail names are the same chemical, and the label's percentage is what matters. Liquid stabilizer is sold pre-dissolved at a lower strength, so it registers sooner but needs more product per ppm.
Aim for 30-50 ppm on an outdoor chlorine pool. That band gives free chlorine real protection from the sun without making your chlorine noticeably weaker. Aim a little higher, usually 60-80 ppm, on a salt pool, because salt cell manufacturers ask for it to protect the chlorine the cell makes.
Size the dose from the gap, not from the bag. Take the target, subtract the current reading, multiply by 0.0834 lb, and multiply again by the pool's gallons divided by 10,000. An 18,000-gallon pool reading 15 ppm that you want at 45 ppm needs 30 ppm of lift: 30 x 0.0834 x 1.8 is about 4.5 lb. Round down rather than up. Stabilizer only leaves the water when water leaves the pool, so a light dose you top up next month costs nothing, and a heavy one costs a partial drain.
Write the dose down with the reading it was based on, and label directions govern over any rule of thumb, including this one. When a pool's history shows the stabilizer reading and the pounds that went in on the same visit, the next tech does not re-dose off a stale number. In PoolBoss a cyanuric acid reading under 30 ppm flags as it is entered, and the dose estimate uses the pool's gallons and the same 13 oz per 10 ppm figure, with a reminder that stabilizer dissolves slowly. That is logging the stabilizer reading against the pool rather than in a notebook on the dashboard.
What each cyanuric acid reading means for your chlorine
The reading tells you how fast the sun will take your chlorine, which is why the action changes by band. Unstabilized chlorine is destroyed by UV quickly; the figure usually cited is 75-90% of free chlorine gone within about two hours of direct summer sun. Stabilizer binds part of the chlorine in a form the sun cannot break down and releases it as the free chlorine is used, which is what lets a weekly dose last a week.
The table below is for an outdoor residential pool. Use it to decide on the stop whether stabilizer belongs in the truck order at all.
| Cyanuric acid | What happens to chlorine | What to do | When to retest |
|---|---|---|---|
| 0-10 ppm | Most of a dose burns off in the first sunny afternoon | Dose to 40 ppm, and add chlorine the same visit | Next weekly visit, 5-7 days |
| 10-29 ppm | Holds overnight, sags by midweek in summer | Top up to 40 ppm | Next weekly visit |
| 30-50 ppm | Target band for a chlorine pool | Nothing; keep free chlorine near 7.5% of the reading | Monthly |
| 60-80 ppm | Target band for a salt pool | Nothing on salt; do not add more on chlorine pools | Monthly |
| Above 80 ppm | Chlorine is increasingly held back | Never add; plan dilution | Monthly |
Why the reading barely moves for a week
Granular stabilizer dissolves slowly, often taking 3-7 days to go fully into solution, so a test the day after dosing reads only part of what you added. This is the single most expensive mistake with conditioner. A tech doses 3 lb, retests two days later, sees 20 ppm where they expected 40, and adds another 3 lb. A week later the pool reads 70 ppm, and the only way back is draining water.
How you add it decides how fast it dissolves and whether any of it is lost. The methods that work are the ones that keep the granules moving in water rather than sitting still:
- Skimmer sock. Pour the dose into a nylon sock or old stocking hung in the skimmer basket and leave the pump running. Water flowing through dissolves it over a day or two. Leave it in until the granules are gone.
- Bucket of warm water. Stir the dose into a 5-gallon bucket of warm water and pour it slowly in front of a return. Not all of it will dissolve; pour the slurry in anyway and brush the floor.
- Not straight onto the floor. Stabilizer is an acid, and granules sitting on plaster for days can etch it or fade a vinyl liner. If a customer has broadcast it, brush it every day until it is gone.
- No backwash for 48-72 hours. Granules that reach a sand or DE filter sit there dissolving, and a backwash throws the undissolved part to waste along with the money.
Low stabilizer is usually why the chlorine is gone by Thursday
If a pool holds chlorine overnight but loses it during sunny days, the cause is almost always low cyanuric acid rather than something in the water eating the chlorine. The two problems look identical on a weekly test, zero free chlorine, and they need opposite fixes: stabilizer for one, a shock and a hunt for algae or organics for the other. Pouring more chlorine into either one only moves the problem to next week.
Separate them with an overnight test. Test free chlorine at dusk after the sun is off the water, then again at dawn before it hits. If the pool drops 1 ppm or less overnight, nothing in the water is consuming chlorine, and the daytime loss is the sun. If it drops more than 1 ppm with no sunlight, the pool has a demand problem that stabilizer will not touch. Most techs can't be at a pool at dusk and dawn, so leave the customer a strip and ask for two readings, or do it on your last and first stops of the day.
Take a two-truck operator running 74 pools across Chandler and Gilbert. After a spring replaster season, five of their accounts had been drained and refilled with fresh water. By June the techs were adding a gallon of liquid chlorine at every visit to those five pools and finding under 1 ppm the following week, while the rest of the route held 3 ppm on less. The drop kits read cyanuric acid below their lowest mark, which on that kit could mean anything from zero to about 20 ppm. On a 15,000-gallon pool, bringing an assumed 10 ppm up to 40 is 30 ppm x 0.0834 x 1.5, about 3.75 lb, and that is what went into each skimmer sock. The tech noted the dose on the visit and did not retest until the next weekly stop, when the pools read 35-40 ppm and held 3 ppm of chlorine through the week on the normal dose.
Getting the low reading right is part of the job. Many drop kits cannot read reliably below 20-30 ppm, strips are least accurate at the low end, and the turbidity test reads high in cold water, so test with the sample near 70°F or warmer. For the details of getting an accurate stabilizer reading, the short version is to treat a reading below the kit's lowest mark as unknown and dose conservatively toward 40 ppm.
Where stabilizer goes, and why it has to go back in
Cyanuric acid does not evaporate or get used up by chlorinating, so a pool only loses it when water leaves or, occasionally, when bacteria break it down. That is why low stabilizer tends to show up on specific pools rather than across a route, and the history usually explains which ones.
Drain and refill is the obvious one: an acid wash, a replaster, or a full drain to fix hard water resets stabilizer to whatever the fill water carries, which is usually zero. Heavy splash-out and backwashing take a few percent at a time. A pool that overflowed through a monsoon or a week of rain can lose 10-20% of its water, and its stabilizer with it. A pool that switched from trichlor tabs to liquid chlorine or a salt cell stops getting the roughly 0.6 ppm of cyanuric acid tabs add for every 1 ppm of chlorine, so it drifts down over a season of backwashes without anyone adding anything.
The last case is rarer and worth knowing. A pool that sat for weeks with no chlorine, a foreclosure or a customer who cancelled and came back, can lose most of its stabilizer to bacteria that break cyanuric acid down. The reading is suddenly low with no drain in the history. Clear the pool first, then restabilize.
When not to add stabilizer
Some pools should never get cyanuric acid, and on others it is already doing more harm than good. Check these before you dose, because once it is in the water, bringing cyanuric acid back down means replacing water, not adding a product.
- Indoor pools. No sunlight means nothing to protect chlorine from, so stabilizer only slows the chlorine down. Keep indoor pools unstabilized unless the pool has large skylights or glass walls.
- Commercial pools under a health code. Many state codes cap cyanuric acid, commonly around 90-100 ppm, and some bar it on indoor pools or spas. Check your state's code before adding it to an HOA or apartment pool.
- Bromine pools and spas. Cyanuric acid stabilizes chlorine only. It does nothing for bromine, so a low reading on a bromine pool is not a problem to fix.
- Pools already fighting chlorine. If a pool will not hold chlorine and the reading is anywhere near the top of the band, more stabilizer makes it worse. Read up on what too much stabilizer does to chlorine before assuming the answer is conditioner.
- Pools on trichlor tabs sitting at 25 ppm. The tabs will add the last few ppm on their own over the next month. A deliberate dose on top is how tab-fed pools end up over 100 ppm by August.
FAQ
Frequently asked questions
Can I add stabilizer and chlorine on the same visit?
Yes, and on a pool reading near zero you should, because the stabilizer will not protect anything for days while it dissolves. Add the chlorine you would normally dose, add the stabilizer through a skimmer sock or bucket, and leave the pool with a normal free chlorine level. Just don't mix them together. Never pour dry stabilizer and a dry chlorine product into the same bucket or the same skimmer, and add them at separate points in the pool a few minutes apart. On the first week expect the chlorine to fade faster than it will once the conditioner is in solution, so a pool that started at 0 ppm cyanuric acid may still read low on the next visit. That is the dissolving lag, not a failed dose. Hold the stabilizer, bring chlorine back up, and judge the result on the second week.
Is it better to use liquid stabilizer instead of granular?
Liquid stabilizer is worth it when you need the reading to count quickly, and granular is the better value for everything else. Liquid conditioner comes pre-dissolved, so it circulates in hours and a retest the next day is meaningful, which helps on a pool opening or a commercial pool that has to show a reading for an inspection. The trade-off is strength: it is a much weaker product by weight than 100% granular, so the same lift can cost several times as much, and heavy jugs take up room on the truck. For weekly residential service, granular through a skimmer sock does the same job for less money, as long as the tech waits a week to retest. Check the label percentage on any liquid product, because the dose math changes with it.
How do I raise cyanuric acid without granular stabilizer?
Stabilized chlorine adds cyanuric acid as a side effect, so switching a pool to trichlor tabs or dichlor for a few weeks raises it slowly without a separate product. Trichlor adds roughly 0.6 ppm of cyanuric acid for every 1 ppm of chlorine it delivers, and dichlor adds about 0.9 ppm. A pool that needs 10 ppm could reach it in a few weeks of normal tab use. This works for small lifts on pools you already service with tabs, but it is slow and hard to stop: the stabilizer keeps climbing as long as the tabs stay in, which is exactly how pools end up in the high-cyanuric-acid problem. Use it for a 10-15 ppm top-up, then switch back to unstabilized chlorine. For a drained and refilled pool starting at zero, dose stabilizer directly.
What cyanuric acid level should a pool have in winter?
Keep the same 30-50 ppm band through winter, because cyanuric acid does not need seasonal adjusting and cannot easily be brought down in spring. Sunlight is weaker in winter and water is colder, so chlorine lasts longer on its own, and a pool at 30 ppm will usually hold fine. The temptation is to let stabilizer slide since chlorine is holding anyway, then find the pool reading 15 ppm in May when the sun comes back. Test it once in late winter, top up toward 40 ppm before the first warm stretch, and let it dissolve before the summer heat arrives. Also remember that cold water makes the turbidity test read high, so a winter reading taken on a 55°F sample can overstate what is really in the water. Warm the sample in the truck first.
How often should I test cyanuric acid on a route?
Once a month is enough for most pools, with an extra test after anything that changes the water. Cyanuric acid moves slowly, so weekly testing mostly repeats the same number, but a monthly reading catches a tab-fed pool climbing or a salt pool drifting down before either affects chlorine. Test it the next visit after a drain and refill, a heavy rain overflow, a filter repair that meant a lot of backwashing, or any time a pool that normally holds chlorine suddenly will not. Test it the visit after you dose, too, to confirm where the pool landed. Logging it on the same form as the weekly readings is what makes a monthly test useful, because the value of a slow-moving number is the trend across months, not a single result.
Why did my cyanuric acid reading go up after it rained?
It almost certainly did not really go up; rain dilutes cyanuric acid, so a higher reading after rain usually points to the test rather than the water. The turbidity test that most kits use reads high when the sample is cold, and a pool cooled by a storm can read 10-20 ppm higher than it did the week before. Undissolved stabilizer from a recent dose finishing going into solution will also push a reading up days later, which can look like the rain did it. Rain water itself carries no stabilizer, so a pool that overflowed during a storm should read the same or lower. Retest with the sample warmed to around 70°F and compare it to the pool's last few readings before you act on it. If the number stays high, look back at what was added in the last two weeks.
