(852) 5923 7263Request a quote
Balanced pool water holds three numbers at once: pH between 7.2 and 7.8, total alkalinity of roughly 80–120 mg/L as calcium carbonate, and calcium hardness of roughly 200–400 mg/L. pH controls how well your chlorine kills and how the water feels on skin and eyes; total alkalinity is the buffer that stops pH from swinging; calcium hardness decides whether the water attacks tile grout and metal or deposits scale. When any of the three drifts out of range, the pool costs more to run: chlorine works harder for less effect, surfaces and equipment wear faster, and swimmers complain. Correct in a set order — alkalinity first, then pH, then hardness.
Pool water has two separate jobs to get right, and people constantly merge them. Disinfection is about killing germs — that is the chlorine. Balance is about the water itself: whether it is comfortable, whether it lets the chlorine do its work, and whether it is quietly attacking or coating everything it touches.
The two are tested separately and fail separately. A pool can hold a perfect chlorine reading while the water is dissolving the grout between the tiles. Another pool can be beautifully balanced and still be under-chlorinated. This article is about the balance side — the three measurements that describe it, and what happens when each one drifts.
pH measures how acidic or alkaline the water is, on a scale where 7 is neutral. For a chlorinated pool the working band is 7.2 to 7.8 — the range recommended by both the WHO's guidelines for swimming pools and the CDC. Most operators aim for the middle, around 7.4 to 7.6. Hong Kong's FEHD licence conditions for public and club pools set a slightly wider band of 7.0–7.8 — see swimming pool licence requirements.
The band is not arbitrary. It is set by two things:
Below 7.2 the water becomes aggressive: it corrodes metal fittings, heaters and handrails, and eats the cement in tile grout. Above 7.8 you get the opposite failure — weak chlorine, cloudiness and a growing tendency to deposit scale.
Total alkalinity is the amount of dissolved bicarbonate and carbonate in the water, expressed as mg/L of calcium carbonate. Its job is to buffer pH — to soak up small additions of acid or alkali so the pH moves slowly instead of jumping.
Every day, things push at your pool's pH: chlorine doses, rain, top-up water, swimmers, carbon dioxide escaping at the surface. Alkalinity is what stands between those pushes and the pH reading.
The CDC's Model Aquatic Health Code accepts 60–180 mg/L; the comfortable operating target for most pools is 80–120 mg/L. Raise it with sodium bicarbonate (alkalinity increaser); lower it with measured acid additions. Note that your chlorine source pushes it too: trichlor tablets are strongly acidic and pull alkalinity down over time, while liquid chlorine nudges pH up — see chlorine tablets vs liquid chlorine.
Calcium hardness is the dissolved calcium in the water, again expressed as calcium carbonate. It changes slowly — nothing in normal operation consumes it — but it decides one important thing: whether your water is hungry for calcium or oversupplied with it.
Water that is low in calcium does not stay low-calcium politely. It takes calcium from wherever it can find it: tile grout, plaster, concrete, mortar. The result is etched, roughened surfaces, failing grout lines and shortened equipment life. Water that is far too high in calcium goes the other way — it deposits scale on tiles, inside pipework, and fastest of all on heat exchanger surfaces, because calcium carbonate becomes less soluble as temperature rises.
Most industry references put the working range for tiled and concrete pools at roughly 200–400 mg/L. Hardness is set mainly by your fill water, so test what actually comes out of the tap rather than assuming — if the fill is soft, new fills will start below range and need raising with calcium chloride. The only practical way to bring high hardness down is partial drain and dilution.
If you want the full theory of when water scales versus when it corrodes, the same index used in cooling water applies to pools: see the Langelier Saturation Index, explained.
| Parameter | Typical target | Acceptable band | Why it matters |
|---|---|---|---|
| pH | 7.4 – 7.6 | 7.2 – 7.8 | Chlorine effectiveness, comfort, corrosion vs scale |
| Total alkalinity | 80 – 120 mg/L as CaCO₃ | 60 – 180 mg/L | Buffers pH against swings |
| Calcium hardness | 200 – 400 mg/L as CaCO₃ | — | Below: etching. Far above: scale |
| Free chlorine | 1 – 3 mg/L | At least 1 mg/L; at least 2 mg/L if stabiliser is used | Disinfection |
| Combined chlorine | As close to zero as possible | — | The "chlorine smell" — see why your pool smells of chlorine |
| Cyanuric acid | 30 – 50 mg/L outdoors; none indoors | Not more than 100 mg/L | Sunlight protection — see cyanuric acid and chlorine lock |
pH, alkalinity, chlorine and cyanuric acid limits per the WHO Guidelines for Safe Recreational Water Environments Vol. 2 and the CDC's 2023 Model Aquatic Health Code. Calcium hardness range as commonly published in pool-industry references; codes generally do not regulate it directly.
Here is the part that costs money. Two pools each hold 2 mg/L of free chlorine. One sits at pH 7.3, the other has drifted to pH 8.1. The first has most of its chlorine in the strong, fast-killing form. The second has perhaps a quarter of it working — the rest is present, measurable, and largely idle.
The operator of the second pool sees algae pressure and cloudy corners, concludes the pool "needs more chlorine", and doses more. The reading goes up; the underlying problem does not move, because the problem was never the amount of chlorine. It was the pH deciding how much of that chlorine was allowed to work. Correcting pH is usually far cheaper than the extra chlorine people pour in to compensate for not correcting it.
The pattern to remember: a pool that "eats chlorine" or "never looks right despite good readings" is usually a balance problem wearing a chlorine costume. Check pH and alkalinity before you raise the dose — and if stabiliser has been accumulating, check that too.
Chlorine costs are the visible loss. The slower, larger loss is the fabric of the pool itself.
Neither failure announces itself quickly. Both are the kind of damage you notice at renovation time, priced in tens of thousands rather than in bags of chemicals.
Between 7.2 and 7.8, and most operators aim for 7.4 to 7.6. In that band chlorine still works well and the water is close to the natural pH of eyes and skin. Below 7.2 the water starts to attack metal and grout; above 7.8 chlorine loses much of its killing power and scale becomes more likely.
Total alkalinity is the amount of dissolved bicarbonate and carbonate in the water, measured as calcium carbonate. It acts as a buffer: it absorbs acid and alkali so the pH moves slowly instead of jumping. With low alkalinity the pH swings with every chemical dose and rain shower. With very high alkalinity the pH drifts upwards and becomes hard to correct.
No. Chlorine disinfects; it does not balance. A pool can hold a perfect chlorine reading while the water is etching the grout or coating the heater in scale. Balance needs its own tests — pH, total alkalinity and calcium hardness — and its own correction chemicals.
Most industry references put the working range for tiled and concrete pools at roughly 200–400 mg/L as calcium carbonate; codes generally do not regulate it directly. Hardness is set mainly by your fill water, so test what comes out of the tap rather than assuming. Raise low hardness with calcium chloride; the only practical way to bring very high hardness down is partial drain and dilution.
Usually because pH, not the dose, is the problem. At pH 8 only around a quarter of your free chlorine is in the strong, fast-killing form, so the reading looks fine while the killing power has quietly left. Check pH and total alkalinity before you raise the dose — and if you dose trichlor tablets, check cyanuric acid too, because accumulated stabiliser also slows chlorine down.
We test pH, alkalinity, hardness, chlorine and stabiliser, and tell you plainly what to correct and in what order.