August 19, 2026

Why won't my pool hold chlorine no matter how much I add?

You dose, you test an hour later, the number looks fine. By the next evening it is back near zero. So you dose harder, maybe shock it, and two days later the strip is white again.

Here is the whole map: free chlorine disappears for exactly two reasons. Either something in the water is genuinely consuming it, or cyanuric acid has moved the goalposts so far that the level you keep "restoring" was never protective to begin with. One cheap test tells you which.

First, separate sun loss from consumption

Some daily loss is normal. Sunlight destroys free chlorine directly, which is why stabilizer (CYA) exists: it is sunscreen for chlorine. A few hours of direct summer sun can strip most of an unstabilized pool's free chlorine, and even a properly stabilized pool gives up a couple of ppm on a bright day. Daytime readings cannot separate sun loss from consumption.

With the sun down, though, a clean pool has almost nothing left that destroys chlorine. So run the overnight loss test:

1. After sundown, dose free chlorine to the top of its target band, pump running.
2. Wait 30 to 60 minutes to mix, then test and note the number.
3. Test again at dawn, before sunlight touches the water.

Loss of about 1 ppm or less overnight means the water itself is clean. Your chlorine is leaving during the day, which points at sun and stabilizer, so skip to the CYA branch below. Loss clearly over 1 ppm with the sun down means something in the water is eating it, and no target adjustment will fix that until you remove the consumer. If your losses happen between dusk and dawn, that pattern has its own causes, covered in chlorine gone overnight.

While you are at it, test combined chlorine (CC) and cyanuric acid (CYA); they decide everything that follows.

One caveat for salt pools: both branches assume the chlorine you think you are adding is actually going in. A cell with scaled or worn plates quietly produces less than its output setting suggests, and that under-production looks exactly like consumption from the outside. If your generator's display and an independent salt test disagree, settle that first: my salt cell says low salt but the water tests fine.

Branch A: something is consuming your chlorine

Algae you cannot see yet

Algae shows up in the chemistry days before it shows up on the walls: a week of invisible cells consuming free chlorine as fast as you add it. The tells are overnight loss well above 1 ppm, demand creeping up for days, walls or steps that feel slightly slick, and often combined chlorine above 0.5 ppm, because chlorine busy fighting organics leaves its spent form behind.

That spent form is why a struggling pool reeks of chlorine: the sharp smell is not too much chlorine but not enough free chlorine finishing the job. If the smell is part of your picture, see why your pool smells like chlorine.

Another normal dose is exactly what the algae has been absorbing all week, so do not bother. Brush everything first, so chlorine can reach cells hiding in the biofilm. Then raise free chlorine well above its normal target, a real shock dose, with the pump running, and keep swimmers out until it settles back into its band. Hold it there, testing and re-dosing often, until the pool passes the overnight test again; stop early and the survivors rebuild into the "green again three days after shocking" story.

Ammonia, the spring-opening monster

A nastier consumer explains the pools that swallow entire jugs in an afternoon and still read zero. When a pool sits with no chlorine for a long stretch, over a winter under a cover or weeks of neglect, bacteria can slowly break cyanuric acid itself down, and one of the end products is ammonia.

The signature is unmistakable: stabilizer that read 60 ppm at closing reads near zero at opening, free chlorine vanishes within minutes to hours of dosing, and combined or total chlorine spikes while free chlorine will not register, because chlorine reacts with ammonia the instant it hits the water. Ammonia is expensive: fully oxidizing it consumes on the order of ten times its own weight in chlorine, so 2 ppm of ammonia can demand around 20 ppm before anything holds.

Two honest options: chlorinate through it with repeated heavy doses of liquid chlorine, pump running, testing every few hours, until free chlorine finally survives; or, when demand is enormous, drain part of the water and refill, which removes ammonia instead of burning money against it. Expect to re-add stabilizer afterward, since the CYA that fed the process is gone.

Branch B: CYA made your target a lie

This is the branch the chemical-aisle signage never explains. Cyanuric acid protects chlorine from the sun by binding it, and the bound portion still shows up on your test as free chlorine, but it is held in reserve rather than actively sanitizing. A little CYA is essential outdoors. Past a point, it binds the chlorine so tightly that what you measure stops doing much work.

That makes a free chlorine number meaningless without the CYA number next to it. At 30 to 50 ppm of stabilizer, roughly 2 to 4 ppm of free chlorine keeps an outdoor pool sanitary. At a CYA of 100 ppm, a reading of 3 ppm is functionally near zero: the active fraction is so small that algae grows almost as if the pool were unchlorinated. You were "holding" chlorine at a level that was never protective, the consumers moved in, and now you are fighting Branch A with a target that guarantees you lose.

The working rule, which the big retail sites will not print because it makes pucks look bad: your minimum free chlorine should sit around 7.5 percent of your CYA reading, with the top of the band near 10 percent. At 40 ppm CYA that is the familiar 3 to 4 ppm; at 68 it is 5.0 to 6.8; at 100 it is 7.5 to 10, which almost nobody runs, which is why almost every CYA-100 pool struggles.

How did CYA get that high? Stabilized chlorine. Every 10 ppm of free chlorine added as trichlor pucks brings roughly 6 ppm of CYA along with it; dichlor granules bring closer to 9. Chlorine burns off and gets consumed; stabilizer does not: nothing uses it up, it does not evaporate, and no chemical on the shelf lowers it. A summer of pucks quietly walks a 40 ppm pool past 100.

The only real lever is dilution: swap out a quarter of the water and you take roughly a quarter off the stabilizer reading. Do it on a cool, overcast day, never all at once where the water table is high, and retest about 24 hours after the refill. Until the number is down, dose with liquid chlorine, which adds no CYA at all.

The short table

One warning: never let concentrated chlorine and acid near each other; they react into a gas you must never breathe. Space additions out with the pump running.

You are done when two things are true: the pool passes the overnight test, and free chlorine sits inside a band actually scaled to your stabilizer, not the generic one on the test-strip bottle. That second condition is the one people lose track of, because the right chlorine target moves every time CYA drifts. It is exactly what Basinly was built to hold onto for you: its CYA-aware chlorine mode rescales your free chlorine target from your logged stabilizer reading, at 100 ppm CYA it asks for 7.5 to 10 ppm instead of pretending 2 to 4 is fine, the water health score reflects that adjusted band, and the charts show the decay curve after every dose, so consumption that would take three frustrating weekends to notice shows up as a line falling faster than it should.