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Field guide · Water problems

Stabiliser too high: when chlorine reads fine but doesn’t work

Stabiliser protects chlorine from the sun, but it also slows chlorine down. Let it build up and you get the classic callout: the test kit shows chlorine, but the water isn’t being sanitised properly and algae is harder to control. Three checks tell you whether stabiliser is the problem.

The short answer

Test stabiliser and free chlorine together, because one only means something next to the other. If stabiliser is high, partly drain and refill to dilute it. That’s the only practical fix. Then stay off stabilised chlorine until it’s back in range.

The WA limits

Stabiliser: recommended 20–50 mg/L, no more than 100 mg/L. Minimum free chlorine with stabiliser: 2.0 mg/L below 26 °C, 3.0 mg/L above 26 °C.

Source: WA Health Code of Practice for aquatic facilities. It binds public pools and is the benchmark for home pools. For home pools, Healthy WA advises stabiliser of 30–50 mg/L.

Some other Australian guidelines set a lower maximum of 50 mg/L and higher minimum chlorine.

The three checks, in order

You need a test kit for free chlorine, pH, total alkalinity and stabiliser (test with a LaMotte Insta-Test strip poolside), and some stabiliser-free water for diluting samples.

1

What chlorine is it on?

Trichlor and dichlor build stabiliser up.

2

Test stabiliser properly

Dilute the sample if it reads high.

3

Compare with free chlorine

Is there enough chlorine for that much stabiliser?

1What chlorine is the pool on?

Trichlor (tablets, sticks) and dichlor (granules) carry stabiliser with every dose: dichlor adds roughly 0.9 mg/L of stabiliser for every 1 mg/L of chlorine. Liquid chlorine and calcium hypochlorite contain none.

Stabiliser isn’t used up. It only leaves with splash-out, backwashing or draining, so a pool kept on trichlor or dichlor climbs steadily.

Salt pools. Salt chlorination is the most common way WA home pools are disinfected. If a salt pool’s stabiliser keeps climbing, check what’s being added on top: ongoing trichlor or dichlor keeps raising it.

2Test stabiliser properly

Most stabiliser tests are turbidity tests: the cloudier the sample, the higher the stabiliser. They read up to about 100 mg/L undiluted. At about 100 mg/L or more, dilute the sample with stabiliser-free water, retest, and multiply up as the kit directs. Or bring a sample in for a Spin Touch analysis.

Correct the alkalinity. Stabiliser adds to the total alkalinity reading. Some Australian health guidelines say to deduct 30% of the stabiliser reading from total alkalinity in stabilised outdoor pools. Use the corrected figure in the LSI calculator; see calcium scale and the LSI.

3Compare it with free chlorine

The more stabiliser, the weaker a given free chlorine level becomes. That’s why health codes set a higher minimum free chlorine for stabilised pools (see the WA limits above), and why industry practice is to raise free chlorine as stabiliser rises.

“Chlorine lock” is a trade phrase. Stabiliser binds chlorine loosely and reversibly, but at high levels so little active chlorine is left free that disinfection can become too slow to protect swimmers.

Why the test kit looks fine. The standard (DPD) free chlorine test also counts chlorine held by stabiliser, while germ kill tracks the active part.

Signs it’s stabiliser

  • A shock that doesn’t hold. Some manufacturers advise having stabiliser at 30–50 mg/L before shocking a home pool.
  • ORP-controlled equipment misbehaving. Stabiliser lowers ORP, so the chlorinator can keep pushing chlorine to excessive levels to reach its set point. Some manuals don’t recommend stabiliser with an ORP sensor; if it’s used, keep it below 20 mg/L and turn the set point down.
  • Not much to gain above about 40 mg/L. In strong sunlight, chlorine loss falls from about 35% an hour with no stabiliser to about 2% an hour at 40 mg/L. More adds little further benefit.

The fix: dilute, then change chlorine

Won’t work

Burning it off. Normal or shock chlorine doses won’t remove it.

Works

Partly drain and refill with fresh water.

  • Work out how much to replace: 1 − (target ÷ current). From 80 to 40 mg/L means replacing about half (drain first, then refill).
  • Don’t fully empty an in-ground pool without professional advice. Where the water table is high, groundwater pressure can push an empty pool out of the ground.
  • Then use liquid chlorine or calcium hypochlorite, with no stabilised products, until stabiliser is back in range: for example AquaGUARD AquaFresh 1250 (liquid chlorine) or AquaGUARD AquaFresh 700 (calcium hypochlorite). Follow the label.

Switching chlorine products? See WorkSafe WA’s pool and spa chemicals guidance for safe storage and handling, and follow each product’s label and SDS.

Indoor pools and spas

Healthy WA doesn’t recommend stabiliser in spas or any indoor pool. With little sun to protect against, it only slows the chlorine.

When it isn’t stabiliser

Final step

Keep testing stabiliser. WA’s Code requires public pools to test it at least weekly; Healthy WA advises home owners using stabiliser to check it daily. Dose every product to its label.

Based on

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.

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