Why Is My Pool Green? Low vs Normal Chlorine, Decided

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Why is my pool green? If free chlorine is below 1.0 mg/L, it is algae outgrowing the sanitiser. If chlorine reads normal, test a second thing: stabiliser above 50 mg/L, phosphate above 0.5 mg/L, pH above 7.8, or copper that chlorine has oxidised. Test chlorine first; it decides which half of the table you are in.

The short answer

Chlorine below 1.0 mg/L: algae. SA Health says algae in a chlorinated home pool indicates free chlorine is not being properly maintained. Its home range is 1.0–3.0 mg/L.
Stabiliser above 50 mg/L slows chlorine. NSW Health's public-pool advisory says above 50 mg/L ‘the disinfection rate is too slow’. SA Health and HealthyWA keep home pools at 30–50 mg/L.
Phosphate above 0.5 mg/L feeds blooms. NSW Health says ortho-phosphate over 0.5 mg/L is conducive to algal blooms and recommends keeping it under 0.2 mg/L.
At pH 7.8, only 28% of free chlorine is the strong form, against 63% at pH 7.2, per NSW Health's advisory table. A normal chlorine reading at high pH can still lose to algae.
Green but see-through, often right after shocking, can be copper, not algae. Pool-chemical makers' technical notes describe chlorine oxidising dissolved copper to green. Test for metals before adding more chlorine.

Green pool decision table: the chlorine reading first, then the second reading that separates the cause (thresholds read 30 September 2026)
Free chlorine reads	Second reading	What it means	Threshold and source	What to do
Low: below 1.0 mg/L	pH, to check the chlorine will work once raised	Algae is outgrowing the sanitiser	Home pools 1.0–3.0 mg/L (SA Health); stabilised outdoor pools at least 2–3 mg/L by temperature (HealthyWA)	Restore chlorine and pH, brush, vacuum settled algae the next day; find out why chlorine fell
Normal: 1.0–3.0 mg/L	Stabiliser (cyanuric acid) above 50 mg/L	Stabiliser lock: chlorine is present but disinfecting too slowly	Max 50 mg/L (NSW Health, public); 30–50 mg/L (SA Health, HealthyWA, home)	Lower it by replacing part of the water; HealthyWA notes it may mean dumping or backwashing water
Normal	Ortho-phosphate above 0.5 mg/L	Nutrient-fed algae that keeps returning	Over 0.5 mg/L conducive to blooms; keep under 0.2 mg/L (NSW Health)	Remove phosphate; HealthyWA suggests checking phosphorus in your mains supply
Normal	pH above 7.8	Chlorine has lost most of its strength	28% strong form at pH 7.8 vs 63% at 7.2 (NSW Health); home range 7.2–7.6 (SA Health)	Lower pH, then re-test chlorine
Normal or high, water clear but tinted	Copper test positive; tint appeared after shocking or a top-up	Oxidised copper, not algae	Public-pool copper max 2 mg/L (NSW Health); colour mechanism from pool-chemical makers' notes (vendor)	Stop adding chlorine; have metals tested and ask about removing them first

Why is my pool green? Start with one chlorine test

Algae is the answer most of the time, and the free chlorine reading tells you whether it is the whole answer. HealthyWA describes algae as single-celled organisms that grow quickly in sunny, warm conditions and ‘can turn the water in your swimming pool green within a few hours’. SA Health puts it more bluntly for chlorinated home pools: ‘the presence of algae is an indicator that free chlorine is not being properly maintained.’

So test free chlorine with a DPD No. 1 test first:

Below 1.0 mg/L: SA Health's home range is 1.0 to 3.0 mg/L. You are in the top row of the table. The fix is chlorine, and the next question is why it fell.
1.0 to 3.0 mg/L, or higher: something is stopping the chlorine from working, or the colour is not algae. You are in the lower rows, and a second test decides which.

The quotable version: a green pool with low chlorine needs chlorine; a green pool with normal chlorine needs a second test before it gets any more chlorine.

If the chlorine is low and you have a salt chlorinator, the chlorinator is the next thing to test. See why a chlorinator stops making chlorine. If the water is hazy rather than green, why a pool goes cloudy covers the separate causes of cloudiness.

Why is my pool going green when the chlorine is normal?

Three readings explain most of these, and each comes with a published threshold.

Stabiliser too high. Cyanuric acid shields chlorine from sunlight, which is why outdoor pools use it, but it also slows disinfection. NSW Health's public-pool advisory says that above 50 mg/L ‘the disinfection rate is too slow’, and names 30 mg/L as optimal. SA Health and HealthyWA keep home pools between 30 and 50 mg/L. NSW Health discourages continuous use of stabilised chlorine compounds because cyanuric acid can then reach very high levels, and HealthyWA notes that stabiliser can build up so that water has to be dumped or backwashed to lower it.
Phosphate. NSW Health: ortho-phosphate above 0.5 mg/L is conducive to algal blooms, and levels below 0.2 mg/L should be maintained. It adds that fixing circulation problems and removing phosphate may be necessary even where chlorine is kept up. HealthyWA suggests asking a pool technician to check the phosphorus in your mains supply.
pH. NSW Health's table shows the strong disinfecting form falling from 63% of free chlorine at pH 7.2 to 28% at pH 7.8, and says ‘above pH 7.8, free chlorine has lost most of its disinfection power.’ A normal chlorine reading at pH 8 is not a normal chlorine pool.

If chlorine reads normal and the pool is still green, test stabiliser, phosphate and pH before adding more chlorine, because more chlorine fixes none of them.

Green but clear: when the colour is copper, not algae

Algae makes water murky and leaves slime on the walls. A pool that is tinted green but still see-through, especially one that turned green right after it was shocked or topped up, may have dissolved copper that the chlorine has oxidised.

This part rests on vendor sources, and we say so. Technical notes published by a pool-chemical manufacturer list oxidised copper among the causes of green water after chlorine is added. It names copper from tap water, algaecides, heaters or mineral systems, and iron as the usual cause of brown or reddish-brown water. One of its case studies describes a pool that turned green when chlorine was added straight after a copper-based algaecide.

Two public-health points back it up. NSW Health sets 2 mg/L as the maximum copper concentration for public pools, and says pool water containing copper must not be discharged to the street gutter or watercourses. HealthyWA says bore water needs more chemicals, ‘especially if the water contains a lot of iron’.

If the water is green and clear and the chlorine is normal, have it tested for copper and iron before adding more chlorine, because in the vendor's own case study it was chlorine that turned the copper green. A pool shop can run a metals test on a sample.

Clearing an algae-green pool: what the health departments say

For the algae rows of the table, the state guidance is short and practical:

Get chlorine and pH back in range. SA Health: if algae is present, ‘ensure that disinfectant and pH levels are within the recommended range.’
Brush, then vacuum. HealthyWA: brush algae off the surfaces, and ‘the next day, vacuum the settled algae from the floor of your pool – do not try to remove it by running the filter.’
Re-check the balance before anyone swims. HealthyWA says to check total alkalinity, pH and calcium hardness first.
If it keeps coming back, SA Health says algae can also be controlled with an algaecide, and to consult a pool supplier if it persists. NSW Health cautions that copper-based algaecides must not build up.

Algae that returns within days of being cleared is a sign the chlorine supply, the phosphate level or the circulation is the real problem. Circulation comes down to pump hours: SA Health and HealthyWA both say the whole volume should pass through the filter in 6 to 8 hours. How long a pool pump should run works that out for your pool.

When a green pool is an equipment problem

A pool that goes green every summer despite correct dosing usually has an equipment cause: a salt chlorinator that is scaled, worn or undersized, a pump running too few hours, or a filter too small for the volume. Pool equipment explained sets out what each should be specified to.

Where the fix is replacing part of the water, for high stabiliser or badly unbalanced water, check first where it can go. See draining a pool to the sewer in NSW and the WA rules.

The cheapest green pool is the one whose chlorinator, pump and filter are sized for its volume from the start. If you are replacing equipment or planning a new pool, compare pool quotes with the equipment named by model, so the chlorinator's output and the pump's flow can be checked against the pool's size.

Common questions

How fast can a pool turn green?

Within hours in warm, sunny weather. HealthyWA says algae grow quickly in sunny and warm conditions and can turn pool water green within a few hours, and that chlorine helps prevent it. SA Health recommends testing chlorine and pH at least once a day.

Is it safe to swim in a green pool?

No. SA Health says algae in a chlorinated home pool indicates free chlorine is not being properly maintained, and that a domestic pool should not be used when disinfectant or pH is out of range. Clear it, re-test and rebalance first.

What stabiliser level is too high in a pool?

Above 50 mg/L. NSW Health's public pool advisory document sets 50 mg/L as the maximum and says above it the disinfection rate is too slow. SA Health and HealthyWA give 30 to 50 mg/L for home pools.

Can phosphates turn a pool green?

They feed the algae that does. NSW Health says ortho-phosphate above 0.5 mg/L is conducive to algal blooms and recommends keeping it below 0.2 mg/L. HealthyWA suggests having a pool technician check the phosphorus level in your mains water supply.

Why did my pool turn green after I shocked it?

If the water is clear but tinted, dissolved copper may have been oxidised by the chlorine. A pool-chemical manufacturer's technical notes describe exactly this, with copper coming from fill water, algaecides or equipment. Have the water tested for metals before adding more chlorine.

Sources

General information only, current at 2026-09-30. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.