Why your pool is not holding chlorine

The overnight chlorine loss test tells you which: if free chlorine (FC) drops more than 1 ppm between sunset and sunrise, something in the water is using it up.

Overnight chlorine loss test (how to run it)

Sunlight is the biggest normal cause of chlorine loss. Remove sunlight from the test, and any loss that remains comes from something in the water. That's the idea behind the overnight chlorine loss test (OCLT), a method popularized by the Trouble Free Pool community.

1. Make sure FC is at least at the minimum for your CYA (the calculator above shows it). Add chlorine in the afternoon if needed.
2. After sunset, once the pump has run long enough to mix the water, test FC and write it down.
3. Leave the pool uncovered and unused. Don't add chlorine overnight. If you have a salt cell, turn it off for the night.
4. Before sunrise, test FC again with the same kit.

Repeat the test on a second night if the result is borderline. A drop test that reads in 0.5 or 0.2 ppm steps makes the answer much clearer than a strip.

Low CYA and sunlight

Without stabilizer, sunlight breaks down chlorine quickly, and a pool can go from a good reading in the morning to near zero by late afternoon. Cyanuric acid (CYA) shields chlorine from UV. If your CYA reads 0 or below 20 ppm and the overnight test passes, low CYA is the likely cause.

• Most pools chlorinated by hand do well at CYA 30–50. Salt pools usually run at 60–80, following the cell maker's advice.
• Raise CYA gradually. It dissolves slowly, and only replacing water brings it back down.
• Remember that more CYA also raises the chlorine you need. Read what stabilizer does before adding it.

What daily loss costs you in product, for 12.5% liquid chlorine:

Fresh 12.5% liquid chlorine, trade percent. Older or hot-stored jugs deliver less. These are estimates. Always follow the product label. Other products: liquid chlorine and bleach amounts.

If you're adding the 5 ppm row every day, fixing one of the causes below usually costs less than the chlorine.

Hidden algae

Algae can use chlorine long before you see it: in the filter, behind light niches, under ladder treads, in plumbing, or as a thin film on a shady wall. A failed overnight test with clear water is the typical sign. The fix is not a larger daily dose but a shock with a product labeled for shocking pools. The label gives the amount and says when you can swim again. Labels usually list an amount per 10,000 gallons, so a 15,000-gallon pool takes 1.5 times that. Brush everything and clean the filter thoroughly the same day. Then repeat the overnight test: a drop of 1 ppm or less means the algae is gone. If it still fails after a few days of following the label, call a pool professional.

Phosphates feed algae but don't consume chlorine themselves. Removing them can slow regrowth, but it doesn't replace the shock. More on phosphate removers.

Ammonia and heavy bather load

Sweat, urine, sunscreen, leaves, pollen, and fertilizer blown in from the lawn all react with chlorine. A pool party can use days' worth of chlorine in an afternoon. Ammonia is the extreme case: breaking it down can take many times its own concentration in chlorine. Until it's gone, FC seems to vanish as soon as you add it, and the total chlorine (TC) reading often rises. Signs:

• combined chlorine (CC), which is total minus free chlorine, stays above 0.5 ppm;
• FC won't rise even after doses that should have raised it several ppm;
• the pool sat unused or covered for a long time, or runoff got in.

The fix is a shock, dosed from your shock product's label for your pool size. Retest the next day: if CC is still 0.5 ppm or more, repeat only as the label directs. Once FC holds and CC falls, run the overnight test. Don't swim until FC is at or below the label's swim limit, often 4 ppm. Pools opened after a long closing are a common case.

Is "chlorine lock" real?

Not in the way it's usually sold. "Chlorine lock" describes a pool where readings look fine but chlorine doesn't seem to work, or where adding chlorine doesn't raise the reading. What's actually going on is one of these:

• High CYA. Stabilizer binds most free chlorine. The binding is reversible, so it isn't locked, but the free chlorine you need rises with CYA. At CYA 80, the minimum is 6 ppm, not 2. The fix is a higher chlorine target or a partial drain, not a special product. See the FC range for your CYA.
• Chlorine demand. Ammonia or algae is consuming each dose (see the sections above).
• A test error (below).

Old or weak chlorine

Liquid chlorine loses strength in storage, faster in heat. According to PHTA, a 12.5% product stored at 95 °F is down to half strength in roughly 48 days, versus about 180 days at 77 °F. A jug that sat in a hot garage all summer may deliver only a fraction of its labeled strength, so your dose seems to "disappear." Buy fresh chlorine, store it in the shade, and compare the expected rise with a retest 30 minutes after dosing. The app learns your jug's real strength from those retests.

Test errors

• Bleach-out. At high chlorine, the pink color of a DPD test can fade or disappear and look like zero. Pool & Spa News puts the risk above about 10–15 ppm. If a reading of zero doesn't fit what you added, make the sample half pool water and half chlorine-free water (distilled or bottled), retest, and double the result.
• Old reagents or strips. Replace reagents yearly (Taylor's kit card says the same) and keep strips sealed and dry.
• Reading total instead of free chlorine. Many strips show both. Make sure you act on free chlorine.
• Sample from the surface or next to a return. Taylor recommends sampling about 18 inches below the surface.

FAQ

Why is my pool always low on chlorine?

The most common reasons are low CYA (sunlight burns chlorine off during the day), hidden algae or a heavy organic load (the overnight loss test is above 1 ppm), and weak or old chlorine. Run the overnight test first. It tells you which group you're in.

Why is my pool reading zero chlorine?

Either chlorine really is gone, because demand from algae, ammonia, or sunlight used it all, or the test is bleaching out at a very high level. If you just added a large dose, dilute the sample 1:1 with chlorine-free water, retest, and double the result before adding more.

Is chlorine lock real?

Chlorine doesn't get permanently locked. High stabilizer holds more chlorine in reserve, so you need a higher free chlorine level to do the same work. Lower CYA with a partial drain, or keep FC at the minimum for your CYA.

Sources

• PHTA fact sheet, Sodium hypochlorite (strength loss by temperature): phta.org
• Pool & Spa News, Overcoming interferences in water tests (DPD bleach-out and dilution): poolspanews.com
• Taylor Technologies, K-1005 instruction card (sampling depth, reagent replacement): PDF
• CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
• Secondary: summary of the Trouble Free Pool method, including the overnight test: poolchemtracker.com