Pool Stabilizer (Cyanuric Acid) in Los Angeles: Levels and Fixes

Stabilizer, conditioner and cyanuric acid (CYA) are the same chemical sold under three names. It bonds loosely to free chlorine and shields it from ultraviolet light. Without it, a sunny afternoon in the Valley can destroy most of the chlorine in an open pool in two or three hours.

So stabilizer is not optional for an outdoor pool in Los Angeles. The trouble is that it never leaves the water once it is in. It does not evaporate, the filter does not catch it and chlorine does not burn it off. Every pound you add stays until water is replaced, and that is how most pools we take over end up with too much of it.

The level to aim for

• Chlorine pool: 30–50 ppm. Enough to hold chlorine through a July afternoon, low enough that a normal chlorine reading still does its job.
• Salt pool: 60–80 ppm. A salt cell makes chlorine slowly all day, and the sun would take it as fast as the cell produces it. Salt pools carry more stabilizer for that reason.
• Under 20 ppm in summer. Chlorine vanishes between visits. You add it on Sunday and the test shows zero by Tuesday.
• Over 90 ppm. Chlorine is present but slow. Algae can start at a reading that would be fine in a pool with 40.

A shaded pool in Sierra Madre or La Cañada needs less than an open one in Encino, because less sun reaches the water. Stabilizer does nothing at night and nothing indoors.

Why chlorine and stabilizer have to move together

The chlorine held by stabilizer is protected, and it is also slower. The more CYA in the water, the more of your chlorine sits in that protected, sluggish form, so the free chlorine reading has to rise with it. Pool technicians work to a simple ratio: free chlorine should be at least 7.5% of the CYA level.

• At 30 ppm CYA, keep free chlorine at 2–4 ppm.
• At 50 ppm, 4–6 ppm.
• At 80 ppm, 6–8 ppm. That is why a salt pool reads higher than you might expect and is still comfortable to swim in.
• At 120 ppm, you would need 9 ppm or more every day. Few people hold that, which is why pools at this level go green with chlorine still showing on the strip.

People call the last case chlorine lock. Nothing is locked. The chlorine is simply outnumbered by the stabilizer holding it, and adding more of the same tablets makes it worse.

Where the extra stabilizer comes from

Trichlor tablets, the white pucks in floaters and in-line feeders, are about half cyanuric acid by weight. For every 10 ppm of chlorine they add, they leave roughly 6 ppm of CYA behind. Dichlor granules, sold as quick-dissolving shock, leave about 9 ppm for every 10 ppm of chlorine.

In a Valley summer a pool can use 3–4 ppm of chlorine a day. Fed only by tablets, that puts roughly 12–17 ppm of stabilizer into the water every week. A pool that starts June at 40 can be past 100 by September, and nothing about the water looks different until algae appears.

Rain and splash-out dilute it a little. So does backwashing a sand or DE filter. In Southern California, though, a dry year with a floater and an auto-fill topping up evaporation only ever concentrates it. That is the pattern in North Hollywood and across the Valley: the chlorine reading looks right, and the pool goes green anyway.

How to test cyanuric acid

Strips give a colour band that is hard to read above 50, which is exactly the range that matters. A drop kit uses a turbidity test instead: you mix pool water with a reagent that turns it cloudy, then pour it into a tube until a black dot on the bottom disappears. The line where it vanishes is your CYA.

• Read it in daylight with your back to the sun. Direct sun on the tube makes the dot vanish early and the reading comes out high.
• Above 100, dilute the sample. Mix half pool water and half tap water, test, and double the result. Most tubes stop at 100.
• Test monthly, and after a refill. It moves slowly, so weekly is more than you need unless you are raising it.

High CYA also throws off one other reading. Part of it shows up as alkalinity, so a pool with 100 ppm of stabilizer reads about 30 ppm higher on total alkalinity than it really is. Subtract a third of the CYA before you add acid to bring alkalinity down.

How to raise stabilizer

About 13 ounces of granular cyanuric acid raises 10,000 gallons by 10 ppm. A typical 15,000-gallon LA pool going from 10 to 40 ppm needs just under 4 pounds. Liquid stabilizer costs more per pound and works the same day; granules take up to a week to dissolve fully.

1. Add half the dose. Raising is easy and lowering means draining, so approach the target from below.
2. Dissolve granules in a bucket of warm water, or put them in the skimmer basket with the pump running. Granules thrown into the pool sit on the plaster and can stain it.
3. Do not backwash or clean the filter for five to seven days. Undissolved granules are still in it, and you would throw them away.
4. Retest after a week, then add the rest if you need it.

If you switch to liquid chlorine or a salt system after years of tablets, check the CYA before adding any. The pool may already have enough for the next five years.

How to lower it

Replace water. That is the method that works every time, and the arithmetic is simple: to go from 100 to 50 you replace half the pool; from 120 to 40, two thirds. Remove the percentage equal to one minus target divided by current.

• Drain in stages. Take the level down a foot or two, refill, retest, repeat. An empty plaster pool can crack as it dries or lift out of the ground when groundwater is high, and in the hills that risk is real.
• Dechlorinate, then use the sewer clean-out. Pool water cannot go into the street or a storm drain in Los Angeles County. It goes into the sanitary sewer clean-out on your property, the same route as filter backwash.
• Stop the tablets first. Otherwise you are draining against a feeder that is putting it back. Liquid chlorine or cal-hypo add no stabilizer.
• Rebalance after the refill. LA tap water is hard and alkaline, so calcium, alkalinity and pH all move when a third of the pool is new water.

Enzyme products that claim to remove cyanuric acid exist, and they are slow, temperature-dependent and expensive per ppm. For a pool at 150 they are not the answer. Reverse-osmosis trailers that filter the water on site are another route, used where drought rules restrict refilling.

Salt pools are different in one way

A salt cell makes unstabilized chlorine, so a salt pool only gains CYA when someone adds it. It loses it only through water replacement, which means a salt pool that is set right in spring usually stays there. When a salt pool goes cloudy or faintly green, the two things to check are cell output and whether the stabilizer has dropped after rain or crept up because somebody dropped tablets in. Both show up on a salt system inspection. The choice between the two systems is covered in saltwater vs chlorine pools.

What it costs to have it handled

On our weekly visits cyanuric acid is part of the full chemical panel, and stabilizer at normal usage is included in the monthly price, which starts at $159 for a pool up to 15,000 gallons. Bringing a badly overdosed pool back down is quoted separately, because it is a drain and refill rather than a dose. The price list shows both.

If your pool keeps turning green with chlorine showing, send us a photo of the water and the last test result, or call (818) 740-6099. If it has already gone green, the green pool clean-up comes first and the stabilizer gets fixed as part of it.