Pool Water Chemistry Basics for the Pool Owner

Why this matters

Five numbers decide whether your pool water is clear, comfortable and safe to swim in, and whether the plaster, liner and heater last as long as they should. You do not need to become a chemist to keep them in line. You need to know what each one does, which way it tends to drift, and when a reading means "adjust" versus "call your pool pro." You also need a short set of storage and handling rules, because pool chemicals are among the most reactive products most households keep, and the serious accidents with them come from mixing, not from swimming.

This card is written for the owner who tests the water between visits. Your pool pro sets the targets for your pool; this card explains what those targets mean.

The five numbers, in plain words

Pool test kits and strips report in ppm, parts per million: how many parts of something are dissolved in a million parts of water. Small numbers, big effects.

Free chlorine (FC). The chlorine that is available right now to kill germs and break down sweat, sunscreen and leaves. It is used up constantly by sunlight, swimmers and debris, which is why it is the number that moves fastest. Too low and algae and germs get ahead; too high and eyes and skin complain. Some kits also report total chlorine; the difference between total and free is chlorine that has already reacted and is mostly spent.

pH. How acidic or basic the water is, on a scale where 7 is neutral. It controls two things you care about: how well your chlorine works, and whether the water is comfortable. Around the middle of the usual target range, pool water feels about like tears. Higher pH makes the same amount of chlorine noticeably weaker. At a pH of about 7.5, roughly half of your free chlorine is in its strong, fast-acting form; at about 8.0, only around a quarter is. So a pool with "enough" chlorine and a pH of 8 can still grow algae.

Total alkalinity (TA). A buffer: dissolved minerals that resist changes in pH. Too low and the pH swings with every rain or chemical addition. Too high and the pH keeps climbing and is hard to bring down.

Cyanuric acid (CYA), also called stabilizer or conditioner. It shields chlorine from sunlight, which would otherwise burn off most of it on a sunny afternoon. Some is good outdoors. Too much slows the chlorine down, so the pool needs a higher chlorine level to do the same work. Stabilizer does not get used up or evaporate; it only leaves when water leaves, by splash-out, backwashing or partly draining.

Calcium hardness (CH). How much dissolved calcium the water carries. Water that is too low in calcium is aggressive: it pulls calcium out of plaster and grout, leaving them rough and etched. Water that is too high leaves scale, a crusty white or gray deposit on tile lines, in the heater and on the salt cell if you have one, and can turn the water cloudy.

The test card, with an example set of targets

Here is what one owner's card looked like on a Saturday morning, against the targets their pool pro wrote for a plaster pool. These targets are an example only. Yours may differ, especially for a vinyl liner, a fiberglass shell or a salt system, and your pro's numbers are the ones to use.

POOL TEST CARD        Sat 9 a.m.   plaster, 20,000 gal
taken at elbow depth, away from return jets

TEST              RESULT    PRO'S TARGET    STATUS
Free chlorine     0.8 ppm   2 to 4 ppm      LOW
Total chlorine    1.0 ppm   (FC + 0.2 max)  ok
pH                7.9       7.4 to 7.6      HIGH
Total alkalinity  130 ppm   80 to 120 ppm   HIGH
Stabilizer (CYA)  70 ppm    30 to 50 ppm    HIGH
Calcium hardness  320 ppm   250 to 400 ppm  ok

Last week: FC 2.5, pH 7.6, TA 120, CYA 60
Chlorine source: tablets in a floating feeder

Read it top to bottom and it tells a single story, which the worked example below follows through.

Why pH drifts, and which way

In most pools pH creeps up, not down. Knowing why tells you what to watch.

Air drives it up. Pool water holds dissolved carbon dioxide, which is mildly acidic. Anything that splashes or bubbles the water releases that gas: waterfalls, a spa spilling into the pool, deck jets, swimmers, and especially a salt chlorine generator, which bubbles as it works. As the carbon dioxide leaves, pH rises. A pool with a lot of moving water climbs faster than a still one.

High alkalinity makes the climb steadier. The more buffer in the water, the more carbon dioxide it has to give up, and the longer the climb continues. That is why TA and pH are usually high together.

New plaster pushes it up for the first weeks to months as the surface cures.

Some things push it down. Tablet chlorine (the slow-dissolving pucks in a floater or feeder) is acidic and lowers pH a little over time. So does heavy rain in many areas. Acid added on purpose lowers it, of course.

The practical rule: a rising pH is normal, and the fix is adding acid in the amount your pro or the product label gives, not a mystery. A pH that falls week after week is less common and worth mentioning to your pro.

Handling and storing pool chemicals

These rules come before any adjustment, every time.

Never mix pool products together. Not in a bucket, not in the skimmer, not in the feeder, not "just a little." Different chlorine products can react violently with each other, and some combinations can start a fire or an explosion: granular calcium hypochlorite shock and chlorine tablets are a known dangerous pair. Acid mixed with chlorine releases chlorine gas.

Add chemical to water, never water to chemical. When you pre-dissolve a granular product, fill the clean bucket with pool water first, then add the product to it. Pouring water onto a pile of chemical, or onto acid, can make it boil, spatter or release gas.

One product at a time. Add one, with the pump running, and give it the time the label says to circulate before you add the next. Many pros suggest waiting several hours between chlorine and acid, or adding them on different days.

Use a clean, dry scoop and a clean bucket for each product, labeled, and never reuse them for something else. A damp scoop moved from one container to another is how two chemicals meet.

Store them apart. Keep chlorine products and acids in separate places, not on a shelf above one another where a leak can drip down. Keep them cool, dry, out of sun, in their original closed containers, and away from fuel, fertilizer, paint and the lawn mower. Keep them locked or out of reach of children.

Wear eye protection and chemical-resistant gloves when handling, and pour low and slowly, standing upwind, so splashes and fumes go away from you.

If something goes wrong:

• A product splashes in an eye: flush with clean running water for 15 minutes and call Poison Control at 1-800-222-1222.
• On skin: rinse with plenty of water and remove contaminated clothing.
• Swallowed: call Poison Control right away; do not make the person vomit unless told to.
• A container hisses, smokes, gets hot, or you smell a strong chlorine or acid odor after something mixed: get everyone out of the area and away upwind, do not try to clean it up, and call 911. Trouble breathing after breathing fumes is a 911 call.

Testing that gives you numbers worth reading

Take the sample at elbow depth, away from the return jets and the skimmer, with the container upside down until it is under, then turned over.

Test at about the same time of day. Free chlorine is highest in the morning and lowest after a sunny afternoon, so comparing a 9 a.m. reading to a 5 p.m. reading tells you more about the sun than the pool.

Mind the reagents. Test strips and liquid drops lose accuracy with age, heat and moisture. Keep them in a cool, dry place in the house, not the pool shed, and replace them by their date. A very high chlorine level can bleach some test reagents and give a falsely low or blank result; if a reading looks impossible, tell your pro rather than adding more.

Worked example: reading the Saturday card

The setup. The card above, from a 20,000-gallon plaster pool with a waterfall feature and tablets in a floating feeder. The owner swims most days; the pro visits every other week.

Read the chlorine and the pH together. Free chlorine is 0.8 ppm against a target of 2 to 4, so it is short by 1.2 ppm even to reach the bottom of the range. But the pH of 7.9 makes it worse than the 0.8 looks, because at that pH a smaller share of the chlorine is in its strong form than at 7.5. Low chlorine and high pH together is the classic setup for cloudy water or a green tint within a few warm days.

Read the alkalinity as the reason the pH is high. TA is 130 against a top of 120, and the waterfall keeps bubbling carbon dioxide out. That combination pushes pH up day after day. Bringing pH down without addressing alkalinity tends to mean adding acid again next week, and the week after.

Read the stabilizer as the slow problem. CYA has gone from 60 to 70 in a week, 10 ppm in seven days. Tablets add stabilizer every time they add chlorine, and stabilizer only leaves with water. At 70 against a target of 30 to 50, the pool now needs more free chlorine to do the same job, and the number will keep rising as long as tablets are the only chlorine source.

Calcium hardness is fine at 320, inside 250 to 400. Nothing to do.

What the owner did. They did not try to fix all three at once. They texted the card to the pool pro with a photo of the strip against its color chart, and asked: how much acid, and do you want me to add liquid chlorine today? The pro replied with a dose of acid for that pool size, to be added first with the pump running, and liquid chlorine that evening after the sun was off the water. The pro also said the next visit would deal with the stabilizer, by partly draining and refilling, and suggested switching from tablets to liquid chlorine for the rest of the season.

The following Saturday. FC 2.8, pH 7.5, TA 115, CYA 70. Chlorine and pH were back in range, alkalinity had come down slightly with the acid, and stabilizer had held at 70, which is what you would expect with no tablets and no water change yet. That is the honest picture: two numbers fixed by a dose, one that can only be fixed by changing water.

What would change the reading. A salt pool with the same card would point even harder at pH, because the cell bubbles constantly, and the stabilizer climb would not be from tablets. A vinyl liner pool has no plaster to etch, so your pro may set a lower calcium target, and a lower CH reading on that card would not mean "aggressive water" the way it does on plaster.

When to stop adjusting and call your pool pro

• Water is green, cloudy or you cannot see the main drain.
• A number is far outside range and you are not sure of the dose.
• The same number keeps coming back out of range week after week.
• Stabilizer is high, since lowering it means draining water, which can damage some pools if done wrong.
• Anyone gets a rash, eye irritation or feels unwell after swimming.
• The heater, pump or salt cell shows an error.

References

• See related: Pool Chemistry Quick Reference (the tech-facing card your pool pro works from)
• The product label on each chemical, which sets its dose, handling and storage rules
• Poison Control: 1-800-222-1222 in the US
• See related: When to Stop and Call a Licensed Pro Around the House