Why does my pool keep losing chlorine?
Your pool loses chlorine because either the sun is destroying it or something in the water is consuming it, and the amount tells you which. A healthy outdoor pool with stabilizer in range should lose no more than about 1 ppm of free chlorine overnight. If yours drops far more than that, or disappears within hours of dosing, you have a demand problem, not a dosing problem, and adding more chlorine without fixing the cause just burns money.
The most reliable way to tell them apart is the overnight chlorine loss test. Read free chlorine at dusk, note the level, then read it again at first light before the sun hits the water. Sunlight is out of the picture overnight, so any large drop points to something living or organic eating the chlorine. Little to no drop means the daytime loss is the sun, which is a stabilizer problem instead.
Work the causes in order: stabilizer level first because it controls how fast sunlight eats chlorine, then organic demand from algae and debris, then combined chlorine. Start by testing free chlorine, total chlorine, pH, and cyanuric acid so you are reading real numbers rather than guessing.
Is your stabilizer too low?
If your cyanuric acid is below about 30 ppm on an outdoor pool, sunlight is burning your chlorine off within hours and that alone explains the loss. Unstabilized chlorine in direct midday sun can lose roughly half its strength in under an hour, so a pool you dosed in the morning can read zero by dinner. Stabilizer (cyanuric acid) shades the chlorine from UV and holds part of it in reserve, which is what lets a pool keep a residual overnight.
The fix is to bring cyanuric acid into range: 30 to 50 ppm for most chlorine pools, and 60 to 80 ppm for saltwater pools. Test it first, then dose with granular stabilizer using the cyanuric acid calculator to get the amount right. Stabilizer dissolves slowly over a few days, so give it time and retest before adding more. The full rundown is in the pool stabilizer guide.
Is your stabilizer too high?
Too much stabilizer causes the opposite problem, and it is the one people miss. The more cyanuric acid in the water, the more free chlorine you need to keep the same sanitizing strength, so a high-CYA pool can read a normal chlorine level and still be effectively unsanitized. Your test says you have chlorine, but it is bound up and doing almost nothing, and algae takes hold anyway. Many people call this chlorine lock.
This is common on pools chlorinated with trichlor tablets or dichlor shock, because both add cyanuric acid every time you use them, so it climbs all season on its own. Test your CYA, and if it is well above your range, you cannot dose your way out. The only reliable fix is to dilute it with a partial drain and refill, covered in how to lower cyanuric acid. Pouring in more chlorine to chase a high-CYA pool is a losing battle.
Is algae or organic material eating the chlorine?
The most common reason a previously balanced pool suddenly stops holding chlorine is an organic demand, usually early algae you cannot see yet. When free chlorine hits zero for even a day, after a hot stretch, a storm, or a heavy swim day, algae gets a foothold and consumes every bit of chlorine you add faster than you can raise the level. A pool that is cloudy and will not hold chlorine is almost always this.
The overnight loss test confirms it. A drop of more than about 1 ppm from dusk to dawn means something living is feeding on the chlorine. The fix is not a special product; it is raising free chlorine to the shock level for your cyanuric acid and holding it there until the demand is gone. The shock calculator sets that target, and the step-by-step is in how to get rid of pool algae. Keep the pump running and brush daily while you do it.
Phosphates get blamed here a lot, but they are algae food, not the thing eating your chlorine, and chlorine does the real work of clearing a bloom. Do not spend on a phosphate remover expecting it to fix chlorine that will not hold; see the honest take in the phosphates guide. Heavy rain is another trigger, since it dilutes your chemistry and washes in organic matter that raises demand.
Could it be combined chlorine (chloramines)?
If your pool smells strongly of chlorine and irritates swimmers' eyes but the free chlorine reads low, you likely have combined chlorine. That is spent chlorine bound to sweat, sunscreen, and other contaminants, and it is a sign the pool has been overwhelmed rather than under-dosed. The strong smell is the giveaway: clean chlorinated water barely smells at all.
You find it by measuring both free and total chlorine. The gap between them is your combined chlorine, and more than about 0.5 ppm of combined chlorine means the pool needs a shock to break it apart. The difference between the two readings is explained in free versus total chlorine. The fix is to shock the pool hard enough to burn off the chloramines, which is exactly why a pool that is clear but shows no free chlorine usually just needs a proper shock, not more tablets.
How do I get my pool to hold chlorine again?
Fix it in order: test the full panel, correct the stabilizer, shock to clear the demand, then run the filter and retest overnight. Skipping straight to dumping in chlorine is what keeps most people stuck. Start by testing free chlorine, total chlorine, pH, and cyanuric acid so you know which cause you are dealing with.
Get cyanuric acid into range next, adding stabilizer if it is low or draining and refilling if it is too high, because nothing else holds until CYA is right. Bring pH into the 7.4 to 7.6 range with the pH calculator, since high pH weakens chlorine and can look like a holding problem. Then raise free chlorine to the shock level for your CYA and hold it there with the pump running 24 hours a day until the water clears and the demand is gone.
The pool is fixed when free chlorine drops no more than about 1 ppm overnight and combined chlorine is near zero. Once it holds that test, let chlorine settle to your normal target and keep it there with the chlorine calculator. Consistency is the real cure: a small daily dose, tablets in a feeder, or a salt generator that never lets free chlorine reach zero beats big weekly swings that let algae restart between doses.
Frequently asked questions
Why is my pool not holding chlorine?
One of four things is usually eating it. Too little stabilizer lets the sun burn chlorine off within hours; too much stabilizer makes the chlorine you have ineffective; early algae or organic matter consumes it faster than you can add it; or combined chlorine (chloramines) means the pool needs a hard shock. Test free chlorine, total chlorine, pH, and cyanuric acid to see which one it is, then fix that cause rather than just adding more chlorine.
How do I get my pool to hold chlorine?
Test first, then get cyanuric acid into range (30 to 50 ppm for chlorine pools, 60 to 80 for saltwater), correct pH to 7.4 to 7.6, and shock the pool to the level for your CYA to clear whatever is eating the chlorine. Run the pump around the clock and hold that level until free chlorine drops no more than about 1 ppm overnight. Then keep it steady with tablets, a salt generator, or a small daily dose so it never hits zero.
Why does my chlorine keep disappearing?
Either the sun is destroying it or something in the water is consuming it. Run the overnight chlorine loss test: read free chlorine at dusk and again at dawn before the sun hits. A big drop overnight means algae or organic demand, so you shock. Little drop overnight but heavy loss during the day means your stabilizer is too low and the sun is burning chlorine off, so you raise cyanuric acid into range.
Why won't my pool hold chlorine after shocking?
Usually because you stopped too soon or your cyanuric acid is off. Algae and organic demand take repeated dosing to fully clear, so keep free chlorine at the shock level for your CYA and hold it, retesting several times a day, until it holds overnight. If cyanuric acid is very high, the shock level becomes hard to reach and the better move is to lower CYA with a partial drain first. A quick single shock that drops back to zero means the demand is still there.
How many times a week should I add chlorine to my pool?
It depends on how you chlorinate. A pool on trichlor tablets or a salt generator feeds chlorine continuously and may need little hand dosing, while a pool dosed by hand with liquid chlorine usually needs topping up every day or two in summer. The real target is simple: never let free chlorine reach zero for your cyanuric acid level. Test two to three times a week and dose to keep a steady residual rather than swinging between high and empty.