1Scale or etching?
- Scale is a hard, rough deposit on surfaces, fittings and pipes.
- On a salt chlorinator it shows as white scale on the electrodes, especially in high-calcium water, and cuts the cell's output. See salt chlorinator low output.
- Hardness salts coming out of solution can make water cloudy when it is otherwise clean.
- Etching is the opposite. Water short of calcium for its other levels (LSI below −0.5) can etch surfaces, corrode fittings and damage concrete and tile grout. Rough surface and a negative LSI? Suspect etching, not scale.
| LSI | What to expect |
|---|---|
| Above +0.5 | Scale-forming. Calcium deposits and scaling. |
| −0.5 to +0.5 | Acceptable range, with 0 as the ideal. |
| Below −0.5 | Aggressive. Corrosion or etching. |
2Run the LSI
LSI = pH + TF + AF + CF − 12.1, with the temperature, alkalinity and calcium factors from a table. Our LSI calculator does the lookups for you. Aim for 0; keep it between −0.5 and +0.5.
What pushes it up: higher pH, alkalinity and temperature (temperature matters least). High alkalinity keeps pushing pH up, and calcium hypochlorite adds calcium with every dose.
Salt pools and the 12.1. The 12.1 is an average correction for total dissolved solids (TDS). Some Australian health guidelines say to switch to the industry table once TDS is above 1,500 mg/L, and almost every salt pool is well above that. In the industry table the constant rises with TDS: 12.1 below 800 mg/L, up to 12.4 for 2,900–5,500 mg/L and 12.5 above 5,500 mg/L. Using 12.1 on a salt pool overstates its LSI by roughly 0.3.
Stabiliser. Stabiliser (cyanuric acid) affects the alkalinity reading. Correct the alkalinity before you work out the LSI; see stabiliser too high.
Worked example: pH 7.4, alkalinity 100 mg/L and calcium hardness 250 mg/L at 29 °C gives an LSI of 0. The same pool at 1,000 mg/L calcium gives +0.6, and scale is likely to form.
3Fixing it
- pH and alkalinity: hydrochloric acid (AquaGUARD Hydrochloric Acid Balance 5) or dry acid (Sodium Bisulphate) lowers pH and total alkalinity. Dose to the label.
- Calcium: partly drain and refill with lower-calcium water. Most "calcium reducer" products chelate calcium rather than remove it: the test reading drops while the calcium stays in the water, so check what the product actually does before you rely on it. If a pool on calcium hypochlorite has excess calcium, WA Health's code suggests switching to a non-calcium chlorine.
- Scaled cell: clean it to the cell manufacturer's instructions.
Acid safety. Dilute acid into water, never water into acid. Keep acid, scale removers and cell cleaners away from liquid chlorine: WorkSafe WA lists them as incompatible, and they can react if mixed.
Calcium hardness targets
There's no single right level; it depends on the pH and alkalinity it sits with, and published ranges differ:
| Source | Concrete / tiled | Other surfaces |
|---|---|---|
| WA Health code (public pools) | 50–400 mg/L for all surfaces | |
| Manufacturers (ranges vary) | 200–275, 200–300 or 200–400 mg/L | 100–225 mg/L; fibreglass 150–250 mg/L |
The WA Health range is set for public aquatic facilities. For a home pool, go by the surface maker's range, and on a salt pool the chlorinator manual.
Perth scheme water
Water Corporation annual means, 01/07/2024–30/06/2025, for selected sampling localities:
| Locality | Hardness (mg/L) | Alkalinity (mg/L) | pH |
|---|---|---|---|
| Two Rocks | 228 | 195 | 7.57 |
| Yanchep | 198 | 163 | 7.63 |
| Neerabup | 190 | 155 | 7.61 |
| Mirrabooka | 124 | 58 | 7.13 |
| Mandurah | 50 | 49 | 8.39 |
| Dwellingup | 29 | 11 | 6.93 |
Water Corporation's hardness counts calcium and magnesium together, so a pool calcium-hardness test reads lower. Two Rocks' high hardness is characteristic of its source water. Treat the table as a guide and test the fill water before you drain and refill.
Bore water. Bore-water top-ups can carry high hardness; some chlorinator manuals say pools on bore or ground water may need the cell cleaned as often as weekly. If total hardness is very high (over 800 ppm), check whether the pool is a mineral pool or filled from a bore.
When it isn't scale
- Cloudy and dirty: look at filtration and bather load. See cloudy pool water and diagnosing dirt in a pool.
- Cloudy but clean, balance right: suspect fine air in the water. Check coagulant dosing, the filter air release, and for air leaks on the pump's suction side.
- Rough surface, negative LSI: suspect etching rather than scale.
- A coloured mark, not a white deposit: see identifying pool stains.
Final step
Retest and re-run the LSI. Note the fill water's numbers on the job card, so next time you know whether the calcium is coming from the tap, the chlorine or the bore.
Based on
- WA Health, Code of Practice for aquatic facilities (s 5.1 and Appendix 7, water balance)
- Water Corporation, Perth drinking water quality data 2024–25 and Hard water
- WorkSafe WA, Getting chemicals for your pool or spa?
- Australian state health guidelines for aquatic facilities
- Industry water-balance references (TDS correction table)
- Pool equipment manufacturers' installation and operating manuals, and product information
Checked against these and other manufacturer and trade sources, October 2026. Always follow the manufacturer's manual and the product label.
General information for pool and spa trade professionals. Every pool is different. This guide does not replace the product label, the SDS, the equipment manual or advice from a licensed tradesperson, and figures quoted from the WA Code of Practice apply to public aquatic facilities. To the extent permitted by law, AGent Sales & Services Pty Ltd accepts no liability for loss or damage arising from reliance on it. Website terms.