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How to balance pool water: alkalinity, pH and chlorine

Fix alkalinity first, then pH, then hardness, and change one thing at a time. Most "my pH won't stay put" problems are really alkalinity problems.

Always follow the label of the product you are using. The dose rates below are guides only.

Why balance matters

Water that is out of balance:

• stings eyes and dries skin;
• eats into marbelite, grout and metal if it is corrosive, or leaves chalky crusts and cloudy water if it is scaling;
• weakens the chlorine, so the pool grows algae even when the test kit shows "enough".

The targets for an outdoor home pool

• Total alkalinity: 80 to 120 mg/L.
• pH: 7.2 to 7.6.
• Calcium hardness: 200 to 400 mg/L for marbelite, and about 150 to 250 mg/L for fibreglass and vinyl.
• Stabiliser (cyanuric acid): 30 to 50 mg/L.

The right order

1. Test everything first.
2. Bring total alkalinity into range. Let the water circulate for several hours, then retest.
3. Adjust the pH. Circulate and retest.
4. Adjust calcium hardness, if needed.
5. Adjust stabiliser, if needed.
6. Bring the chlorine to its target.

Add one chemical at a time, and wait at least as long as the label says before adding the next.

Total alkalinity

Alkalinity is the shock absorber for pH.

• Too low: the pH swings with every dose, rainstorm or swim, and the water becomes corrosive.
• Too high: the pH creeps up and is hard to bring down, and the water tends to scale and go cloudy.

To raise it, add sodium bicarbonate. As a guide, about 170 g for every 10 000 litres raises alkalinity by 10 mg/L. Dissolve it in a bucket of pool water first.

Lowering alkalinity is slow. It takes small doses of acid over a week or two.

pH

At pH 7.5, about half of the chlorine is in its strong, fast-killing form. At pH 8.0, only about a quarter is.

• To lower pH: use pool acid or dry acid, in small steps. As a cautious step, add no more than about 250 mL of pool acid for every 10 000 litres at a time. Pour it slowly into the deep end with the pump running, wait several hours and retest.
• To raise pH: use soda ash, as the label directs.

Liquid chlorine, calcium hypochlorite and salt chlorinators push the pH up. Trichlor tablets and rain push it down. On the Highveld, most pools drift upwards and need acid regularly.

Calcium hardness

Water that is too soft dissolves calcium out of marbelite, leaving it rough and pitted. Water that is too hard leaves white crusts at the waterline.

To raise hardness, add calcium chloride. About 140 g for every 10 000 litres raises it by 10 mg/L. It gets hot as it dissolves, so add it slowly to a bucket of water, never water onto it. There is no simple chemical to lower hardness. The fix is to dilute the pool with softer water.

Stabiliser

Sunlight destroys unprotected chlorine within hours. Stabiliser shields it.

• Too low: the chlorine disappears on sunny days.
• Too high, above about 100 mg/L: it locks up the chlorine. It builds up from trichlor tablets and never breaks down. The only fix is fresh water.

About 100 g for every 10 000 litres raises it by 10 mg/L.

Corrosive or scaling?

• Corrosive water (low pH, alkalinity and hardness): stinging eyes, etched marbelite, and blue or green stains from corroded copper.
• Scaling water (high pH, alkalinity and hardness): white crusts, cloudy water and rough walls.

Working out a dose

Take the guide rate, scale it to your pool's volume, and multiply by the number of steps you need. For a big change, add it in two goes and test in between.

Handle chemicals safely

Never mix pool chemicals with each other. Add chemicals to water, never water to chemicals, and store them locked away from children.

This guide is based on Chapter 4 of Swimming Pool Maintenance in South Africa.