Pool water chemistry without the guesswork
Most pool problems people try to solve with more chlorine are not chlorine problems. They are balance problems, and chlorine cannot fix them.
Test free chlorine, pH, total alkalinity, calcium hardness and cyanuric acid. Hold pH between 7.4 and 7.6, total alkalinity between 80 and 120 ppm, calcium hardness between 200 and 400 ppm for a plaster pool, and cyanuric acid between 30 and 50 ppm. Get those right and chlorine does its job on a fraction of the dose — get them wrong and no amount of chlorine will hold the water.
pH is the master control
Chlorine's killing power depends on pH, and dramatically so. At a pH of 7.5 the chlorine in your pool is roughly half in its active form; at 8.0 that drops to about a fifth. The same dose does a quarter of the work. This is why a pool that has drifted high looks cloudy and unresponsive no matter how much shock goes in.
The 7.4 to 7.6 window is also where the water is comfortable for eyes and skin and least aggressive toward plaster and equipment. If you only manage one number consistently, manage this one.
Alkalinity is what stops pH moving
Total alkalinity is the buffer. With it between 80 and 120 ppm, pH stays roughly where you put it. Below that, pH bounces on every rainfall and every bather load — the phenomenon known as pH bounce, and the reason some pools seem to need adjusting constantly.
The sequence matters: correct alkalinity first, then pH. Adjusting pH in a pool with no buffer is work you will repeat in three days.
Calcium hardness, and why plaster pools care
Water that is short of calcium will go and find some — out of the plaster, the grout and the tile. That is what etching is. For a gunite pool with a plaster or aggregate finish, hold calcium hardness between 200 and 400 ppm. Too high and you get scale on the tile and in the heater instead. Vinyl and fiberglass pools are more forgiving here because there is no cementitious surface to dissolve.
Cyanuric acid: sunscreen with a limit
Cyanuric acid protects chlorine from ultraviolet light. Without it, an outdoor pool in a Tennessee July can lose most of its free chlorine in a single day. With too much, it binds chlorine so tightly that the sanitiser is present on a test strip but not doing much — often called chlorine lock.
Thirty to fifty ppm is the useful range for an outdoor pool. The thing to know is that cyanuric acid does not evaporate or break down meaningfully; it only leaves by draining and replacing water. So it accumulates, particularly if you use stabilised tablets, and the only fix for a high reading is dilution.
A routine that actually holds
Consistency beats intensity. A pool tested twice a week and nudged is far easier to keep than one tested monthly and corrected hard.
- Twice weekly — free chlorine and pH
- Weekly — total alkalinity
- Monthly — calcium hardness and cyanuric acid
- Test before adding anything, and add one thing at a time with circulation running
- Retest the following day rather than the same hour — most chemicals need a full turnover to read true
When to stop and call someone
Persistently cloudy water after balancing, a surface that feels rough or chalky, staining that will not brush out, or chlorine demand that keeps climbing are all signs of something the test kit is not showing — high combined chloramines, metals in the fill water, or a filtration problem. Those are worth a professional look before you spend more on chemicals.
What does a pool actually cost here?
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Build yours and see the rangeCommon questions
- What should pool pH be, and why does it matter so much?
- Between 7.4 and 7.6. Chlorine's effectiveness depends heavily on pH — around 7.5 roughly half the free chlorine is in its active killing form, but at pH 8.0 that falls to about a fifth, so the same dose does a quarter of the work. That is why a pool with drifted pH looks cloudy and unresponsive no matter how much shock is added. It is also the range that is most comfortable for swimmers and least aggressive toward plaster and equipment.
- What is chlorine lock, and how do you fix it?
- It is what happens when cyanuric acid — the stabiliser that protects chlorine from sunlight — accumulates too high and binds the chlorine so tightly that it registers on a test but sanitises poorly. Cyanuric acid does not evaporate or break down, so it only builds up, especially with stabilised tablets. The only real fix is dilution: partially drain and refill to bring the level back into the 30 to 50 ppm range.
- How often should I test my pool water?
- Free chlorine and pH twice a week, total alkalinity weekly, and calcium hardness and cyanuric acid monthly. Consistency matters more than intensity — a pool nudged twice a week stays easy, while one tested monthly and corrected hard swings between problems. Always test before adding anything, add one product at a time with the pump running, and retest the next day rather than the same hour.