You add chlorine and it is gone by the next day, or your pH keeps climbing no matter how much acid you pour in. Pool chemistry is a handful of numbers that pull on each other, so fixing one in the wrong order can knock another out of range. This page explains what each number does, which ones go wrong, the targets from CDC and the test kit makers, and when to get help.
What keeps pool water balanced?
Think of it as two jobs. Chlorine keeps the water safe, and the balance numbers (pH, alkalinity and calcium) keep the chlorine working and the pool surfaces from wearing away.
- Free chlorine kills germs. Free chlorine is the part still able to work, measured in ppm (parts per million). Combined chlorine is chlorine that is used up; it smells and stings eyes. Total chlorine minus free chlorine equals combined chlorine.
- pH sets how strong chlorine is. pH measures how acidic or basic the water is. CDC says chlorine kills germs less well as pH goes up, especially above 8.0.
- Total alkalinity holds pH steady. Total alkalinity (TA) is the water’s buffer. Too little and pH bounces around; too much and Taylor calls the water “overbuffered,” so pH is hard to move.
- Cyanuric acid shields chlorine from the sun. Cyanuric acid (CYA), also called stabilizer, protects chlorine outdoors. Pentair notes the sun’s UV destroys 90% of unstabilized chlorine within two hours.
- Calcium hardness protects surfaces. Hayward explains that too little calcium makes water corrosive and too much causes scale.
Which pool chemistry levels go wrong, and what are the signs?
| Problem | What it does to the water | How to tell |
|---|---|---|
| Low free chlorine | Lets germs and algae grow | Free chlorine under 1 ppm, green tint, slimy walls |
| High combined chlorine | Strong chlorine smell, red eyes | Total chlorine reads well above free chlorine |
| High pH | Weakens chlorine, builds scale, clouds water | pH above 7.8 |
| Low pH | Corrodes metal, etches plaster, burns off chlorine fast, stings eyes | pH below 7.2; a yellow pH test |
| High alkalinity | pH keeps creeping up; water may cloud and scale | TA above 120 ppm |
| Low alkalinity | pH swings with every chemical you add | TA below 80 ppm |
| High CYA | Locks up chlorine so it works slowly | CYA above 50 ppm; algae with a normal chlorine reading |
What should I check first when chlorine won’t hold or pH won’t go down?
- Your test kit. Check the dates on reagents and strips. CDC notes strips are less accurate than a DPD drop kit. Do not test right after a shock; LaMotte says to wait.
- Free vs total chlorine. If total is much higher than free, the pool needs a shock to burn off combined chlorine. A normal daily dose will not clear it.
- CYA. Too little and the sun eats your chlorine in hours. Too much and the chlorine works slowly. Both feel like “chlorine won’t hold.”
- Algae food. Pentair warns that nitrates and phosphates can create extremely high chlorine demand, in some cases dropping chlorine to zero. Hidden algae does the same; see our green and cloudy pool water hub.
- Total alkalinity, when pH won’t drop. High TA keeps pushing pH back up. Lower TA with acid in steps and retest.
- What you add. Liquid chlorine and cal-hypo are strongly basic, and a salt cell makes sodium hydroxide, which Taylor says pushes pH higher. Pools fed this way need acid more often.
Safety first. Never mix chlorine and acid, or any two pool chemicals. CDC warns that pool chemicals can injure people when mixed, and acid plus chlorine can release chlorine gas. Always add acid to water, never water to acid. Wear goggles and gloves, open one container at a time, and keep swimmers out while you dose. Store chemicals apart from each other, dry, below 95°F, and locked away from kids and pets.
Pool chemistry numbers to know
These targets come from CDC’s home pool guidance and Model Aquatic Health Code (MAHC), and from the test kit makers Taylor Technologies and LaMotte.
| Reading | Target | Source |
|---|---|---|
| Free chlorine | At least 1 ppm, or at least 2 ppm with stabilizer; 2 to 4 ppm is ideal | CDC; Taylor |
| Combined chlorine | Under 0.4 ppm; shock when it tops 0.2 ppm | CDC MAHC; Taylor; LaMotte |
| pH | 7.0 to 7.8; 7.4 to 7.6 is ideal | CDC; Taylor |
| Total alkalinity | 80 to 120 ppm ideal; 60 minimum, 180 maximum | Taylor |
| Calcium hardness | 200 to 400 ppm | Taylor |
| Cyanuric acid | 30 to 50 ppm ideal; public pools capped at 90 ppm | Taylor; CDC MAHC 2023 |
| CYA added by trichlor | 7 ppm per pound in 10,000 gallons | Taylor |
| Chlorine test limit | Above 10 ppm, the test can bleach out | CDC; Taylor |
CYA reads as part of your alkalinity test. Taylor says to subtract one third of your CYA reading from your TA reading to get the true number.
Liquid chlorine vs cal-hypo vs trichlor: how each one moves your numbers
The chlorine you buy changes your other readings, which is why two pools with the same chlorine level can need very different care.
- Liquid chlorine (sodium hypochlorite). Strongly basic, so it pushes pH up. It adds no CYA and no calcium.
- Cal-hypo (calcium hypochlorite). Also basic, and it adds calcium with every dose. Watch hardness in hard-water areas.
- Trichlor tablets. Acidic, so pH and alkalinity drift down. Each pound also adds stabilizer, and CYA only comes down when you drain and refill.
LaMotte sets the alkalinity target to match: 80 to 120 ppm for pools on liquid chlorine or cal-hypo, and 100 to 150 ppm for pools on trichlor or dichlor. We suggest picking one main chlorine type and testing weekly, so you learn which way your pool drifts. Salt pools behave like liquid chlorine pools; our salt water pool problems hub covers the extras.
When should you call a pool or spa pro?
- Chlorine still won’t hold. If CYA, pH and alkalinity are in range and chlorine still vanishes, a full lab test can find phosphates, nitrates or metals.
- CYA or calcium far too high. The fix is usually a partial drain, and draining can damage some pools.
- Scale, etching or stains. Rough plaster, white crust or colored stains mean the water has been out of balance for a while.
- Automatic feeders. Acid pumps, chlorine feeders and controllers that read wrong need a service visit.
Pool pros bill by the visit or by the hour, so write down your last few test results before they arrive.
Common questions
Do I adjust alkalinity or pH first?
Adjust total alkalinity first. Taylor says to treat TA first because it controls the other balance numbers, and Hayward lists TA before pH in its setup order. Once TA is in range, fine-tune pH and retest after each change.
Does shocking a pool raise pH?
It depends on the shock. Liquid chlorine and cal-hypo are strongly basic and push pH up, while trichlor is acidic. Wait for the water to mix, then test pH and correct it.
Should I add chlorine if pH is high or low?
If free chlorine is low, add it either way, because the pool needs protection. Then fix pH: at high pH, chlorine works poorly, and at low pH, LaMotte notes it burns off quickly. Lower a high pH with acid; raise a low one with soda ash.
How do I bring pH down in a pool quickly?
Add muriatic acid, following the label for your pool’s size, with the pump running so it mixes. Retest after the water has circulated, and add more in small steps rather than one big dose. If you have a salt chlorinator, Hayward and Pentair discourage dry acid in dry regions, because it can leave by-products that damage the cell.
Is 8.2 pH too high for a pool?
Yes. CDC’s range tops out at 7.8, and chlorine loses much of its strength above 8.0. Also, 8.2 is the highest color on most phenol red test kits, so the real pH may be higher.
Does baking soda raise pH or alkalinity more?
Baking soda (sodium bicarbonate) mainly raises alkalinity. Taylor notes it also raises pH slightly. To raise pH itself, Hayward and LaMotte call for soda ash (sodium carbonate).
Why does my pool pH test show yellow?
A yellow result means the pH is at or below the lowest color on the kit, which is 6.8 on most phenol red tests. The water is acidic and needs soda ash. If the test turns purple instead, chlorine above 10 ppm may be throwing off the reading.