Pool Water Chemistry Basics for Australian Homeowners

pH

pH measures how acidic or alkaline the water is. The ideal range is 7.2–7.6. Water that is too acidic (low pH) can corrode equipment and irritate skin. Water that is too alkaline (high pH) reduces chlorine effectiveness and can cause scaling.

pH is the most important parameter to manage because it affects almost everything else — chlorine efficacy, calcium behaviour, and equipment wear.

Free chlorine

Free chlorine is the amount of sanitiser available to kill bacteria and algae. For most residential pools, the target is 1–3 ppm (parts per million). Chlorine is consumed by sunlight, bather load, and organic matter — it needs to be replenished regularly.

Total alkalinity

Alkalinity is the water's buffer against pH swings. The target range is 80–120 ppm. Low alkalinity makes pH unstable and hard to manage. High alkalinity can cause scale and cloudy water. Adjust alkalinity before fine-tuning pH.

Cyanuric acid (stabiliser)

Stabiliser protects chlorine from being destroyed by UV sunlight. For outdoor pools in Australia, the target is 30–50 ppm. Without stabiliser, chlorine breaks down rapidly in direct sun. Too much stabiliser reduces chlorine effectiveness — a common issue in pools that are frequently topped up with pre-stabilised chlorine.

Calcium hardness

Calcium hardness measures dissolved calcium in the water. The target range is 200–300 ppm (slightly higher for concrete pools). Low calcium can etch plaster and damage fibreglass surfaces. High calcium causes scale buildup on tiles, equipment, and pool walls.

Water chemistry is a system, not a checklist. Key relationships:

• pH and chlorine — chlorine works most effectively between pH 7.2 and 7.6. Higher pH significantly reduces sanitiser power.
• Alkalinity and pH — alkalinity buffers pH. Fix alkalinity first when both are out of range.
• Stabiliser and chlorine — stabiliser protects chlorine but too much locks it up. If chlorine readings are consistently low despite dosing, check stabiliser levels.
• Calcium and pH — in concrete pools, calcium and pH are in a slow chemical relationship. Low pH dissolves calcium from the plaster surface; high pH deposits it as scale.

Test strips are convenient for routine checks. Liquid test kits provide more accuracy. Professional water testing at a pool shop is useful for detailed analysis or when something is not right.

• To lower pH: add muriatic acid or sodium bisulphate
• To raise pH: add sodium carbonate (soda ash)

Always add chemicals to water, never water to chemicals. Distribute around the pool perimeter. Re-test after circulation and before adding more.

Liquid chlorine

Fast-acting, no stabiliser added, easy to dose. Degraded by sunlight — best applied in the evening. The most common primary sanitiser for Australian pools.

Granular chlorine (cal-hypo or dichlor)

Cal-hypo adds calcium — useful for low-calcium water but can scale. Dichlor adds stabiliser — convenient but can lead to stabiliser buildup over time.

Salt chlorinator

A salt system generates chlorine from salt dissolved in the pool water. Lower chlorine smell, consistent dosing, but the cell requires periodic cleaning and replacement. The upfront cost is higher but ongoing chemical costs can be lower.

Tablet feeders

Slow-dissolving tablets in a floating feeder or inline chlorinator. Convenient for consistent dosing. Most tablets are dichlor or trichlor — both add stabiliser, so monitor cyanuric acid levels.

Green water (algae)

Usually a chlorine deficiency. Shock the pool (raise free chlorine to 10+ ppm), brush the walls, run the filter for 24–48 hours, and test to find the root cause — often insufficient regular dosing or a stabiliser issue.

Cloudy water

Can indicate poor filtration, imbalance in chemistry, or high bather load. Test all parameters, clean or backwash the filter, and ensure the pump is running adequate hours (typically 6–8 hours daily in summer).

Scaling or calcium deposits

High pH, high calcium, or both. Lower pH gradually, and consider a sequestering agent. On concrete pools, persistent scaling may indicate the water is aggressive.

Chlorine smell

Strong chlorine smell usually indicates chloramines — combined chlorine from reaction with sweat and organic matter — not too much chlorine. Shock the pool to break apart chloramines.

Different surfaces interact with chemistry differently:

• Concrete/render — directly affected by pH and calcium. Acidic water dissolves the surface; alkaline water deposits scale. Requires more careful chemistry management.
• Fibreglass — less reactive, but still affected by extreme pH levels and chlorine exposure. Avoid calcium buildup on the surface.
• Tile — grout lines are sensitive to pH and calcium. Tile surfaces themselves are highly resistant.