In the evening you measure 2 mg/l of free chlorine, in the morning zero — and the water still looks clean. A healthy pool does not behave like that: overnight, with no rain, no sunshine and nobody swimming, the drop in free chlorine should not exceed roughly 1 mg/l. When it does, something is consuming the chlorine, and this article shows how to tell the six most common causes apart by measuring rather than guessing.
What counts as normal chlorine loss and what does not
Chlorine is always being used up in a pool — that is its job. What you need to separate, though, are two completely different routes by which it leaves the water.
- Loss to sunlight (photolysis). The UV component of sunlight breaks hypochlorite down directly. In unstabilised water in direct sun, the half-life of free chlorine is on the order of tens of minutes — published estimates range roughly between 17 and 45 minutes depending on the intensity of the radiation. This loss is what the stabiliser is there to handle.
- Loss to consumption (chlorine demand). Chlorine reacts with whatever is in the water: sweat, urine, cosmetics, pollen, leaves, algae, bacteria. This part carries on in the dark as well.
During the day, then, a drop of 30–50 % of the dose is perfectly normal for an outdoor pool in summer. At night the solar component disappears, which makes the night an ideal diagnostic window: anything that consumes chlorine at night is organic or microbial contamination. An overnight loss of up to 1 mg/l is considered normal; a larger loss means there is something extra in the water.
The overnight chlorine loss test: the cheapest diagnostic you have
The overnight chlorine loss test (OCLT in English-language sources) gets almost no attention, yet it splits the causes into two large groups over a single night and costs nothing more than two measurements.
- In the evening after sunset (and at least 30 minutes after the last chemical addition, so it has time to mix in) measure the free chlorine. Use a method that resolves tenths — a photometer or a drop test; strips are far too coarse for this.
- Cover the pool with a cover or a slatted roller cover if you have one. Leave the filtration running in its usual mode and add nothing.
- In the morning before sunrise (or before direct light reaches the surface) measure again, using the same method.
The interpretation is simple: a loss of up to 1 mg/l = the water is clean, so look for the problem in the stabiliser, the pH or the way the pool is run. A loss above 1 mg/l = there is an organic or biological load in the water, typically algae in the early stage. Always repeat the test using the same procedure, otherwise the numbers are not comparable. We describe the measuring methodology and the differences between tester types in detail in the article on how to test pool and hot tub water properly.
Cause 1: cyanuric acid — too little, or conversely too much
Cyanuric acid (CYA), sold as chlorine stabiliser, binds chlorine into a UV-resistant compound and releases it gradually. Without it, summer chlorine is gone by mid-morning. With an excess, the opposite problem arises.
Low CYA (below 20–30 mg/l). The sun “burns” the chlorine off. You typically recognise this because the value is fine in the morning and zero in the afternoon, while the overnight loss test comes out well. The fix is to top up the stabiliser to the target level — the ANSI/PHTA standard gives an ideal range of 30–50 mg/l for conventionally chlorinated pools.
High CYA (above ~100 mg/l). Here the chlorine is not lost, it simply stops working. The more cyanuric acid there is, the greater the share of chlorine held in the “reservoir” and the lower the concentration of active hypochlorous acid that actually disinfects. The water then turns green even at a measured 2 mg/l of free chlorine, and the owner is under the impression that the chlorine is disappearing — in reality it is just doing nothing, and it is being consumed by the algae that have taken off. This is why the FC/CYA ratio is used: free chlorine should be roughly 5–7.5 % of the CYA value (so with CYA at 40 mg/l, about 2–3 mg/l of chlorine).
A crucial fact: cyanuric acid cannot be removed from the water by dosing chemicals or by filtration. It only breaks down very slowly, biologically. The only reliable answer to an excess is dilution — a partial water change. If you measure 150 mg/l and want to get down to 50, you have to replace roughly two thirds of the volume. We go through the background and the dosing in the article on chlorine stabiliser and cyanuric acid.
CYA rises on its own if you dose stabilised products (trichlor tablets, dichlor granules) — every kilogram of trichlor adds around 0.6 kg of cyanuric acid to the water. That is why measuring CYA at the start of the season and once during it is basic hygiene, not a luxury.
Cause 2: chlorine demand after a heavy load
Chlorine demand is the amount of chlorine the water “swallows” before any measurable free chlorine appears in it at all. After a heavier load it can be so high that a normal dose vanishes without a trace.
- Thunderstorms and torrential rain. It is not just dilution: rainwater brings in dust, pollen, nitrogen compounds and run-off from the surroundings. In detail in the article on rain, storms and pool chemistry.
- Lots of bathers. Every person brings sweat, skin oils, cosmetics and sunscreen residue into the water. Chlorine forms chloramines with them — combined chlorine, which does not disinfect and smells.
- High water temperature. Above 28 °C both reaction rates and microbial multiplication increase; chlorine consumption rises noticeably.
The distinguishing sign: the loss is a one-off, tied in time to a specific event, and after a shock dose the water returns to normal and then holds. If the chlorine is still disappearing several days after the shock, look for the cause elsewhere.
Cause 3: early-stage algae that you cannot see yet
The most common reason for a bad overnight test result. Algae start growing long before the water turns green — and even at this stage they consume chlorine at a rate that normal dosing cannot keep up with.
The subtle signs that appear before the colour does:
- Slippery walls. Run a finger along the wall below the waterline in a corner or behind the ladder. A healthy surface “squeaks”, biofilm is smoothly slippery.
- Dull water. It does not lose its blue, it just stops being “sharp” — you can see the bottom, but the pattern of the liner loses contrast.
- Chlorine holds for an hour and then drops. The classic picture: the dose disappears faster than the sun would account for.
- Cloudiness persists even after changing or cleaning the filter.
The answer is a shock dose of unstabilised chlorine (sodium or calcium hypochlorite — they do not raise CYA), maintained at an elevated level for as long as it takes for the overnight test to come in below 1 mg/l, plus mechanical brushing of the walls and floor. We describe the types and prevention in the article on pool algae: types and prevention, and the acute situation in the text on green pool water.
Cause 4: high pH and chlorine that only “looks” like chlorine
Free chlorine exists in water in two forms: hypochlorous acid (HOCl), which disinfects effectively, and the hypochlorite ion (OCl⁻), which is an order of magnitude weaker. The ratio between them is set by the pH — and it is a steep relationship.
| Water pH | Share of hypochlorous acid (approximately, 25 °C) | Practical impact |
|---|---|---|
| 7.0 | ~75 % | Very effective, but the water is aggressive towards surfaces |
| 7.2 | ~66 % | Optimum for both effectiveness and comfort |
| 7.5 | ~50 % | Still fine, the upper limit of the recommended band |
| 7.8 | ~33 % | Effectiveness falls markedly, the chlorine “cannot keep up” |
| 8.0 | ~24 % | Chlorine is largely ineffective, risk of limescale |
The recommended operating band is pH 7.0–7.4 (Czech and European sources give up to 7.6 as acceptable). At pH 8.0 you therefore have the same 2 mg/l of chlorine on paper as your neighbour, but roughly a third of the disinfecting power — and algae that will consume that chlorine for you. High pH does not itself make chlorine disappear; it does, however, mean the chlorine cannot defend itself, and consumption then shoots up. That is why pH is measured first, before chlorine, and adjusted with a pH minus product. Watch out for aeration: counter-current jets, water features and a salt chlorinator all raise the pH systematically.
Cause 5: organic matter in the pool
Leaves on the bottom, a film of pollen on the surface, grass cuttings blown in by the wind, bird droppings — every gram of organic matter is extra work for the chlorine. Grass and leaves also release nitrogen compounds and phosphates into the water before you manage to fish them out.
The distinguishing sign: chlorine consumption correlates with the weather and the surroundings (wind, a birch in flower, pines above the pool) and improves visibly after a thorough vacuum of the floor and a clean-out of both the skimmer and pump baskets. The article on leaves, pollen and insects in the pool summarises the practical procedures.
Cause 6: phosphates and nitrates from the surroundings
Phosphates and nitrates do not consume chlorine directly — they are nutrients for algae. They get into the pool from lawn fertilisers, from irrigation water, from leaves and from some phosphonate-based anti-limescale products. A well in an agricultural area can have considerably more nitrates than mains supply.
The practical view: if you keep the chlorine and the filtration in order, algae will not take over even at higher phosphate levels. The problem arises where chlorine repeatedly collapses — then phosphates act as fuel that turns every lapse straight into cloudiness. They are removed with phosphate removers (lanthanum or aluminium salts); the precipitate has to be filtered out and the filter then backwashed. For nitrates there is no practical chemical remover — the only way is dilution.
Diagnostic table: test → measured value → meaning → what to do
| Test | What you measure | What it means | What to do |
|---|---|---|---|
| Overnight chlorine loss | up to 1 mg/l | The water is biologically clean | Look for the cause in CYA, pH or filtration run time |
| Overnight chlorine loss | above 1 mg/l | Algae or organic load | Shock dose + brushing, repeat the test |
| Cyanuric acid | below 20 mg/l | Chlorine has no protection against UV | Top the stabiliser up to 30–50 mg/l |
| Cyanuric acid | above 100 mg/l | Chlorine is “asleep”, effectiveness falls | Partial water change, switch to unstabilised chlorine |
| pH | above 7.6 | Chlorine loses its disinfecting power | pH minus down to 7.0–7.4, only then deal with chlorine |
| Free vs. total chlorine | difference above 0.3 mg/l | Combined chlorine (chloramines) after a load | Shock, aerate, extend filtration |
| Look and feel | slippery walls, dull water | Biofilm and early-stage algae | Brush mechanically, shock |
| Filter pressure | 0.3–0.5 bar above the baseline | Clogged media, reduced flow | Backwash the filter, check run time |
What to do straight away when chlorine will not hold
- Measure the pH and adjust it first. Without that, any chlorine dose is partly wasted.
- Measure the cyanuric acid. It decides whether topping up the chlorine makes sense or whether the water needs diluting.
- Shock with unstabilised chlorine — sodium or calcium hypochlorite, not tablets. Shocking with a stabilised product raises CYA and deepens the problem.
- Vacuum the floor and brush the walls, including the corners, the steps and the area around the jets. Chemicals alone are not enough against biofilm without mechanical disruption.
- Check the filtration. Run time, pressure on the gauge, condition of the media, cleanliness of the baskets. The filtration time you need for your volume and pump output is covered in the article on how long to run the pool filter.
- Repeat the overnight test after 24 hours and only carry on according to the result.
Prevention: so it does not happen again
- Cover the pool. A solar cover or a slatted roller cover limits UV breakdown of chlorine and at the same time keeps leaves and pollen out. The difference in chemical consumption is noticeable within a single week.
- Measure regularly, not only when there is a problem. Free chlorine and pH twice a week, CYA at the start of the season and then once in mid-summer.
- Keep the filtration time matched to the volume and extend it on hot days, do not shorten it.
- Alternate your chlorine source. If you dose trichlor tablets all season, expect CYA to climb — replace some of the doses with hypochlorite.
- Shower before swimming. The cheapest measure against chlorine demand there is.
When a partial water change is the only answer
Chemistry cannot do everything. The water has to be partially replaced if at least one of the following applies:
- Cyanuric acid exceeds roughly 100 mg/l and you cannot keep chlorine at the corresponding ratio.
- The water has been “oversalted” for a long time — a high content of dissolved solids (TDS) from repeated dosing, showing up as dull water and chemicals that do not respond.
- Nitrates are high and algae keep coming back even with correctly managed chlorine.
- A combination of high hardness and high alkalinity that you cannot bring down.
Replacing 30–50 % of the volume is usually enough; never drain the whole pool without verifying the conditions, especially with liner tanks and where the groundwater table is high. The target concentration is easy to calculate: after replacing half the volume, both the CYA and the TDS value drop to roughly half.
Frequently asked questions
How do I tell the difference between “chlorine is being lost” and “chlorine is not working”?
Loss means the value falls towards zero. Not working means you can measure the chlorine but the water still goes cloudy or green — typically at a pH above 7.8 or with cyanuric acid above 100 mg/l. That is exactly why you measure both, not just the chlorine.
Can the overnight test come out badly because of a salt chlorinator or a UV lamp?
No, as long as you switch them off for the night or leave them in their normal mode and do not change it between the two measurements. The test compares two values under the same conditions — consistency is what matters, not exactly what is running.
How much chlorine should I dose when CYA is high?
Work with the ratio: free chlorine approximately 5–7.5 % of the CYA value. At CYA of 80 mg/l that means roughly 4–6 mg/l of chlorine, which is uncomfortable for swimming — so it is better to bring the CYA down by dilution than to raise the dose.
Will an algaecide help when chlorine will not hold?
An algaecide is a preventive product, not a rescue one. At the point where algae are already consuming the chlorine, the decisive steps are a shock dose and mechanical cleaning. An algaecide only makes sense afterwards, in the maintenance phase.
Do I have to cover the pool for the overnight loss test?
You do not, provided you measure before sunrise and the pool is in shade. Covering it does make the result more precise, though — it rules out falling leaves, insects and any early-morning light.
Disappearing chlorine is almost always a signal, not a fault. The overnight loss test, the pH value and the cyanuric acid value narrow six possible causes down to one over a single night — and save you both money and litres of needlessly dosed product. Testers, unstabilised chlorine, phosphate removers and stabiliser are all in the water treatment category; further diagnostic guides are in the Guide section.
