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pH control of your swimming pool

  • Balancing your pool-water is a vital part of keeping a healthy pool environment.
  • The disinfection system you choose will have complimentary products to control your pH.
  • A correct balance is achieved when the level of acidity and the level of alkalinity closely match the natural levels.
  • This relationship is known as the pH value and is measured on a scale from 0-14 with 7 being neutral.


High pH value – Alkaline conditions

  • A high pH value (above 7) indicates a higher content of alkaline substances than acid types in the water.
  • If the pH value is too high, then the effectiveness of the disinfection process (when using chlorine) is reduced. Scale formation may occur, the water could become cloudy and the capability of the filter might be reduced due to blockage.

Low pH value – Acid conditions

  • A low pH value (less than 7) indicates a greater acidic than alkaline content.
  • If the pH value is too low, it is probable that corrosion will occur around the pool fittings.
  • Most importantly a low PH may lead to skin and eye irritation.



Ideal pH

  • As you can see, though your pool water may appear clean you can only be certain that it is safe and healthy if you regularly check (and if necessary re-adjust) the pH value.
  • The ideal pH value is 7.2 – 7.6.

0 1 2 3 4 5 6
7.2 to 7.6
8 9 10 11 12 13 14

Adjust pH as follows:

Add to Water
Below 7.2
0.5kg/45m3(cubed) (10,000 gals)
Above 7.6
0.5kg/45m3(cubed) (10,000 gals)

Test water after 24 hours, repeat as necessary until desired level is achieved.

Water hardness


As suggested in the introduction, the water you use to fill your pool (tap water) surprisingly may not be ideal swimming pool water.

Tap water varies greatly depending on where you live. For example, water in Scotland, where the ground is predominantly hard granite rock, is very ‘pure. Hard rock will not dissolve quickly on contact with rain water so the water remains relatively unaffected, just picking up minor mineral content, after it hits the ground. This soft water will generally have a low pH value (that is to say higher acidity) and will lead to corrosion if left untreated.

In Kent, however, with its chalky white rock, rain water picks up ‘impurities’ such as calcium salts which will raise the alkalinity of the water. This hard water with its higher alkalinity content can cause cloudiness and limescale formation.

Calcium hardness

Hardness is the measurement of the amount of calcium salts present in the water. It is nly expressed in milligrams per litre (mg/l) – the concentration of calcium in the water.

Water hardness is also increased by water evaporation. When water evaporates any dissolved minerals (like calcium carbonate) will remain, increased in concentration (higher ppm, or mg/ltr). Calcium hardness is important because of what is known as water’s ‘hunger’ for calcium. If there is not enough calcium in the water a level below 200 mg/l (ppm) then the water will try to find calcium from the pools surroundings and fitting. This may cause corrosion problems.

On the other hand, too much calcium in the water will lead to will lead to calcium deposits (scale formation). Your ideal calcium hardness level should be between 200 and 500 mg/l (ppm).


Hardness - mg/l (ppm)
Over 300
Description of water
Moderately soft
Slightly hard
Moderately hard
Very hard


Too soft
1. Check TA and increase to 80 – 140 mg/l (ppm) if necessary. (See below).
2. Add 0.7kg per 45³ (10,000 gallons) to increase hardness by approx. 10mg/l (ppm)

Too hard

1. Check TA and decrease to 80 – 140 mg/l (ppm) if necessary. (See below).
2. If hardness persists – part empty pool and refill. Check calcium hardness on refilling the pool and if necessary repeat the process.