Chlorine gone overnight - what is happening in my pool?
During the day, chlorine has an alibi. Sunlight destroys free chlorine, stabilizer only slows the process down, and a hot afternoon can honestly account for a couple of ppm. So when the reading is lower every morning, most owners shrug and blame the sun.
The sun sets. That is the whole trick. Between dusk and dawn, UV loss is zero, and a clean pool loses almost nothing overnight - a few tenths of a ppm to slow background oxidation, no more. So if you dose in the evening and a meaningful chunk of that chlorine is gone before sunrise, the sun did not take it. Something in the water consumed it, and chlorine being consumed means chlorine doing work: oxidizing algae you cannot see yet, ammonia left over from a stabilizer collapse, or a heavy organic load. That is what the overnight chlorine loss test (OCLT) measures, and it is the cleanest diagnostic in pool chemistry because it removes the biggest variable entirely.
The rule of thumb: overnight loss of 1.0 ppm or less is a pass. More than that, and you have a consumer to find. If your pool passes overnight but still bleeds chlorine during the day, the problem lives on the sunlight side - stabilizer level, dosing timing, demand from bather load - and that diagnosis is covered in why your pool won't hold chlorine.
How to run the test properly
The test is simple, but it is easy to run in a way that produces garbage. The point is to isolate the hours with no sun, so both measurements have to happen in the dark.
- Dose in the evening, after direct sun is off the water. Bring free chlorine up to the top of your band - for a typical outdoor pool at 30 to 50 ppm of stabilizer, that band is roughly 2 to 4 ppm. Evening dosing is good practice anyway, since it gives chlorine a full night to work before the sun starts taxing it.
- Run the pump for at least 30 to 60 minutes after dosing. You are testing the pool, not the deep end near the return jet. Unmixed chlorine reads high where you added it and proves nothing.
- Test free chlorine and combined chlorine after full dark. Write both down, with the time.
- Test again at dawn, before any sun touches the water. Not "before breakfast" - before sunlight. Ten minutes of morning sun on the surface is enough to muddy a marginal result.
- Subtract. Dusk free chlorine minus dawn free chlorine is your overnight loss.
One honest caveat about test kits: a 1 ppm threshold demands a test that can actually resolve 1 ppm. Color-comparator blocks that step 1, 3, 5 cannot tell a 0.5 ppm loss from a 1.5 ppm one. A FAS-DPD drop test reads to 0.5 or 0.2 ppm and is the right tool here. Strips are not in the conversation.
Pick a dry, calm night. Rain and wind wash fresh organic material into the pool overnight, which is real chlorine demand but not the demand you are trying to measure. And a swim party the same evening loads the water with sweat and sunscreen - run the test on a night the pool rested.
Reading the result
| Overnight loss | What it points to |
|---|---|
| 1.0 ppm or less | Pass. Nothing significant is living in the water |
| 1 to 2 ppm | Early demand: algae getting started, biofilm, organic load |
| 2 to 5 ppm | An active bloom, even if the water still looks acceptable |
| Most or all of it | A full bloom, or ammonia after a stabilizer collapse |
A pass: 1.0 ppm or less
Your sanitizer is winning and the water is biologically quiet. If daytime readings still sag, stop shocking the pool and start looking at stabilizer and sun exposure, because you have just proven the loss happens only when the sun is up.
1 to 2 ppm: something is getting started
This is the most valuable result the test can give you, because it catches trouble before it is visible. Algae consumes chlorine long before it tints the water; by the time a pool looks green, the bloom is weeks old. A repeatable 1.5 ppm overnight loss in clear water usually means algae establishing itself somewhere the circulation is weak - behind ladder treads, behind a light niche, in the folds of steps - or a biofilm doing the same thing. Check the combined chlorine numbers from your two tests: combined chlorine climbing overnight is chlorine reacting with organic material, and it corroborates the diagnosis. Anything above about 0.5 ppm combined is worth clearing on its own.
2 to 5 ppm: an active bloom
Losses this size mean a substantial living population, whatever the water looks like. Run a hand along the wall below the waterline - slick or slimy means algae even when the water reads clear. Water in the low 80s F (27 to 29 C) accelerates all of this, which is why these failures cluster in late summer.
Nearly everything gone: think ammonia
If you put in 4 ppm at dusk and wake to a reading near zero, night after night, one specific failure belongs at the top of the list: ammonia after a stabilizer collapse. When a pool sits without chlorine for weeks - a winter under a cover, a vacation, an equipment failure - bacteria can consume the cyanuric acid itself and leave ammonia behind. The signature is a spring opening where stabilizer tests far lower than it did at closing while chlorine vanishes almost as fast as you add it, because ammonia consumes roughly ten times its own weight in chlorine before free chlorine can hold at all. A cheap aquarium ammonia test confirms it. The fix is brute force: keep adding measured doses of chlorine and retesting every hour or so until the demand finally breaks - it can take several times what the pool normally needs in a week - and only then rebuild stabilizer and rebalance.
What to do when you fail
Whatever the size of the loss, the response is the same shape: kill the consumer, then verify.
- Raise free chlorine and hold it there. One evening dose is not a fight; a bloom eats it and carries on. Test and re-dose morning and evening so the level stays up continuously.
- Run the pump around the clock while you are fighting. Chlorine only kills what it reaches.
- Brush everything - walls, floor, steps, behind the ladder - daily. Brushing breaks algae and biofilm off surfaces and puts it where the chlorine is.
- Repeat the overnight test each night. The loss number shrinking is your progress bar. You are done when the pool loses 1.0 ppm or less overnight and combined chlorine sits under 0.5 ppm - not when the water merely looks better, because looking better is exactly the state from which blooms rebound.
Then go fix whatever let chlorine get low enough for the problem to start, which is usually a daytime story - the stabilizer and dosing territory covered in the pillar post.
The test's one practical annoyance is bookkeeping: two readings twelve hours apart, across several nights, is exactly the kind of data a notebook loses. Basinly is the pool care app we built for this - the + button on the chlorine chart logs a single reading in seconds, so the dusk and dawn pair land in your history and the chart shows the overnight drop shrinking night over night, with each dose you added marked on the same line. Watching that delta close is how you know the pool is actually clean, not just clear.