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How to look after pool water: test first, then fix in order

Test first, then add only what a reading asks for, in the order that does not undo itself: alkalinity, then pH, then chlorine. How to take a sample, read a kit, fix one thing at a time and know when the water is ready.

The shelf in the pool aisle has a simple answer to looking after a pool: five products, every week. The water rarely needs all five, and adding a chemical it does not need costs money and can push another reading out of range.

The alternative is just as simple and costs less. Test the water, read each number against its range, and fix only what is off, one thing at a time and in the right order. Then retest before adding anything else or getting in.

The pool care loopSample and test, then read the results. If every reading is in range the water is ready and you wait for the next scheduled test. If not, fix one thing, wait, and retest — which takes you back to reading the results.Sample and testElbow deep, mid-poolKit, not guessworkRead and logEach reading againstits bandAll inrange?Fix one thingAlkalinity, pH,then chlorineWait, then retestLet the pumpcirculate itSwim-readyChlorine and pHboth in rangenoyesnext scheduled testretest, then read the new result
Every path ends in a fresh test. Nothing is added because of the day of the week.
  1. Step 1

    Get a kit you can trust

    A DPD drop kit for chlorine and pH; strips for a quick check in between.

    CDC names a DPD test kit as the most accurate way for a home pool owner to measure chlorine and pH. Strips are fine for a quick check between kit tests, and CDC says plainly that they are less accurate.

    Whatever you use, follow its own instructions every time, and keep the reagents cool, dry and out of the sun. Old reagents give confident wrong answers, so replace them each season.

    More: Ways to measure free chlorine · Ways to measure pH

  2. Step 2

    Take a good sample

    Elbow deep, away from the return jets, and test it straight away.

    Most bad readings start with the sample, not the kit. Push a clean plastic bottle mouth-down to about elbow depth, then turn it upright to fill. That is below the few inches the sun reaches, which read low on chlorine.

    Take it where the floor starts to slope toward the deep end, away from the return jets, which carry freshly treated water, and away from corners and steps, where water barely moves. In a hot tub, turn the jets and blowers off first. Dip a strip in the sample, not the pool.

    Where to take a pool water sampleA pool in cross-section. Take the sample at elbow depth, about 18 inches, where the floor starts to slope toward the deep end. Avoid the sunlit top few inches, the water right at a return jet, and corners and steps where water does not circulate.Top few inches: sunlight changes the readingReturn jet: skipSample here: elbow depth, about 18 inwhere the floor starts to slopeCorners, steps: dead waterShallow endDeep endHot tub: turn the jets and blowers off first. Test straight away either way.
  3. Step 3

    Run the test and read it

    Free chlorine and pH first; they decide whether the pool is usable today.

    Fill to the line with the bottom of the curved surface at eye level, mix by turning the tube over rather than shaking, and match colours in daylight. On a drop-count test, keep adding drops until the colour stops changing.

    If free chlorine reads zero straight after a large dose of chlorine, suspect the test rather than the pool: very high chlorine can bleach the colour out. The free chlorine guide says how to dilute the sample and retest.

    If you track your pool here, log the result. Each reading is judged against its range for your pool, and anything out of range explains itself on the page.

    More: Free chlorine · Combined chlorine

  4. Step 4

    Fix one thing at a time, in order

    Alkalinity, then pH, then chlorine. Add one chemical, circulate, retest.

    Total alkalinity holds pH steady, so a pH correction made before an alkalinity fix drifts as soon as alkalinity moves. pH decides how well chlorine works: as pH rises, chlorine kills germs more slowly. So chlorine goes last, into water it can work in.

    Add one chemical, let the pump circulate it, and retest before the next. The first correction moves the others — soda ash also raises alkalinity, and acid also lowers it — so a second amount worked out before the retest is an upper limit, not an instruction.

    The order to correct pool waterFirst total alkalinity, which holds pH steady. Then pH, which sets how well chlorine works. Then free chlorine. Between each step, add one chemical, let it circulate, and retest.1 Total alkalinityHolds pH steadyRaise: baking soda (bicarbonate)Lower: acid2 pHSets how well chlorine worksRaise: soda ashLower: acid3 Free chlorineKills germsRaise: liquid chlorine or cal-hypoLower: time and sunlightBetween steps: one chemical at a time, let the pump circulate it, then retest

    More: Total alkalinity · pH

  5. Step 5

    Retest before you add more, or swim

    Work out the next step from the new reading, never from the old one.

    After the water has circulated, test again and decide the next addition from what it reads now. Counting down from the first test is how pools get overdosed.

When the water is ready

Both of these, on the same test:

Free chlorine1 – 4 mg/L

pH7.2 – 7.8 pH

If the pool uses stabilizer (cyanuric acid, or chlorine tablets that contain it), CDC recommends at least 2 ppm free chlorine rather than 1. A pool tracked here gets that floor automatically once a cyanuric acid reading is logged. The top of the free chlorine range is the most allowed with people in the water, so after a shock nobody swims until a test says it has come back down.

Shock when the test says so, not on a weekly basis

Shock when combined chlorine isabove 0.4 mg/L

Combined chlorine is total chlorine minus free chlorine: chlorine that has already reacted with sweat and urine, and that irritates eyes, skin and lungs instead of disinfecting. Above its action level, the code public pools follow calls for breakpoint chlorination, which means adding about ten times the combined chlorine, all at once. Fed in small additions it builds more of what it is meant to destroy.

You need both a free and a total chlorine reading to see it, which most drop kits give you. If you log both here, the dosing page works the amount out for your pool.

Algaecide is optional. Chlorine held in range kills algae on its own, and no health authority we found says a home pool needs algaecide every week. If you use it, follow its label, and never as a substitute for chlorine.

How often to test

CDC says to test chlorine and pH at least twice a day, and more often when a lot of people are using the pool. One test-strip maker's instructions ask for at least twice a week. Aim for CDC's figure whenever the pool is in use.

Alkalinity, calcium hardness and stabilizer change slowly, so they need testing less often than chlorine and pH. When you add a pool here, a weekly test reminder is set up for you. Change it to suit how the pool is used.

Handling pool chemicals safely

  • Read the whole label before each use, and wear goggles and gloves.
  • Never mix pool chemicals with each other. Chlorine products and acid together can make toxic gas.
  • Add chemical to water, never water to chemical. Pre-dissolve only if the label says to.
  • Open one container at a time and close it before opening the next.
  • Never add chemicals while anyone is in the water.

Track your pool here

Log a test and every reading is judged against its range for your pool, the dosing page works out amounts from your pool's volume in the order above, and a weekly reminder keeps the habit going.

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This guide is general information, not a substitute for a product's label or a pool professional.