How to fix a green pool (and why it keeps coming back)

A green pool is an algae bloom, and in a chlorinated pool it almost always means free chlorine has dropped too low for too long. Most green pools can be cleared without draining by getting the pump and filter running, brushing, correcting pH, shocking with chlorine to the product label and filtering until the water turns clear.

Quick answer: how to fix a green pool

To fix a green pool, restore circulation, bring pH into range, then shock with chlorine and keep filtering until the green and the cloudiness are gone. According to SA Health, algae in a chlorinated pool is a sign that free chlorine is not being maintained, so every fix starts with chlorine and pH. The order matters because chlorine works poorly when pH is high and because a stopped filter undoes the chemicals.

1. Check the pump runs and the filter is clean; empty the skimmer and pump baskets.
2. Scoop out leaves and debris, then brush the walls, floor, steps and corners.
3. Test free chlorine, pH and stabiliser, and correct pH to 7.2 to 7.6 first.
4. Shock with chlorine at the dose the product label gives for your pool volume.
5. Run the filter continuously, cleaning or backwashing it as pressure rises.
6. When the water goes cloudy grey, clarify or floc and vacuum, then retest before anyone swims.

For the target numbers behind each step, see our guide to pool chlorine and pH levels.

How green is it? A severity check before you start

How green the pool is decides how much chlorine it will take, how long it will take and whether it is a weekend job or a call to a pool service. Lo-Chlor groups green algae into three practical levels, from a little on the steps to water so thick you cannot see the bottom. Use the table to place your pool. The first two columns follow Lo-Chlor; the last two are our own guidance, not Lo-Chlor’s.

Whatever the level, treat the pool as closed until it is clear and tested, and keep children supervised and the pool gate shut.

Step by step: clearing a green pool

This guide clears a green pool in this order, which is a sequence rather than a rule: circulation, cleaning, pH, chlorine, filtration, then clean-up of the dead algae. Keep an eye on pH throughout, because Lo-Chlor says chlorine is much less effective at high pH.

1. Get water moving. SA Health says that if the pool filter does not operate properly, the chemicals added will not be effective. Empty the skimmer basket and the pump lint pot, and check the pump primes and runs. If it does not, start with our pool pump not working checks.
2. Remove debris and brush. Lift out leaves and sticks, then brush every surface. Lo-Chlor notes that algae grow where circulation is poor (steps, corners, cracks and between pebbles) and that most algae need a surface to cling to, which brushing breaks up.
3. Test and correct pH. SA Health gives a home-pool pH range of 7.2 to 7.6 so chlorine works efficiently. NSW Health explains why: as pH rises, less of the free chlorine is in its strong disinfecting form.
4. Shock with chlorine. SA Health describes shock dosing as raising free chlorine to 10 mg/L and holding it for at least one hour, with the pump and filter running the whole time. Dose by your pool volume and the product label, never a guessed amount. Lo-Chlor notes liquid chlorine adds alkalinity, so retest pH afterwards.
5. Filter non-stop and watch the pressure. Backwash a sand filter or hose a cartridge as pressure climbs; the gauge is your guide.
6. Deal with the dead algae. Chlorine bleaches the algae, leaving the pool cloudy grey, and Lo-Chlor says dead algae is very fine and tends to pass straight through a sand or cartridge filter. A clarifier clumps it so the filter can catch it; a floc settles it on the floor to vacuum to waste.
7. Retest before swimming. Check free chlorine and pH are back in range and the water is clear.

Algaecide or chlorine first? Chlorine first. Lo-Chlor says chlorine acts in hours where algaecide takes days, so algaecide is a follow-up, used to its label once the chlorine has done the heavy work. Can you skip the scrubbing? Not reliably: brushing is part of every sourced method, because algae left clinging to surfaces regrows.

How long does a green pool take to clear?

According to Lo-Chlor’s treatment guidance, a pool with patches of algae can start to look better within hours of a correct shock, but green algae that has been there for three or four months can take up to a week to clear. The time depends on three things: how long the algae has been there, how well the filter copes with the dead algae, and whether pH and stabiliser let the chlorine work. The colour usually goes first; the cloudy grey stage that follows is the slow part, because it relies on filtration and vacuuming rather than chemistry.

There is no honest “overnight” promise. The fastest route is the right order: working pump, clean filter, pH in range, then enough chlorine to clear the green completely. Lo-Chlor’s test is simple: you have added enough chlorine when no green colour is left in the water. Adding more clarifier will not speed things up either; Lo-Chlor warns that overdosing will not improve clarity faster. If a medium or heavy green pool is making little progress despite correct readings and continuous filtering, work through the checks in the next section.

Why is my pool still green after shock?

A pool that stays green after shock usually has something stopping the chlorine from working or from reaching the algae, not a lack of product on the shelf. Lo-Chlor points out that treating algae without fixing the underlying cause means it keeps coming back. Work through these checks in order.

Green but chlorine is high

If the test shows plenty of chlorine and the pool is still green, check pH and stabiliser first: both reduce what that chlorine can do, according to NSW Health. Also confirm the reading is free chlorine: NSW Health notes total chlorine is free plus combined chlorine, so a high total figure can overstate the chlorine that is actually working.

Cloudy after shock, or cloudy with good readings

Cloudy grey water after a shock is usually dead algae waiting to be filtered or vacuumed. Lo-Chlor also notes that high pH can make calcium carbonate drop out of solution, giving a milky look, and that cloudiness from early algae will return unless the algae is treated. Never run floc through a cartridge filter: Lo-Chlor says it must not be filtered through cartridge or DE media.

Green after rain: why subtropical storms tip pools over

Heavy rain turns pools green because it washes in the very things algae feed on, at the time of year algae grow fastest. SA Health lists rain, wind and soil as common ways algae arrive, and names a rainstorm as a reason a pool may need shock dosing. Lo-Chlor adds that when rain runs over garden beds into a pool it carries nitrates, phosphates, metals that can stain and algae spores. On the Sunshine Coast, summer storms, heat and year-round swimming stack these together, and Lo-Chlor notes algae are more prevalent in summer because they need sunlight.

After a storm, skim and empty baskets before the debris sinks and rots, test as soon as the rain stops, and shock if chlorine has dropped. SA Health recommends testing more often in hot sunny weather. If floodwater, not just rain, got into the pool, treat it as contaminated: the Queensland Government says flood-affected pools will contain harmful germs and can become mosquito breeding sites, and that a licensed electrician must inspect any flood-affected pool electrical equipment before you use it.

Is it safe to swim in a green pool?

No. SA Health says a home pool should not be used if the disinfectant level or pH is outside the recommended range, or if the water is dirty or cloudy, and a green pool fails at least the clarity test and almost always the chlorine one. Algae means chlorine is too low to reliably control germs, and the same SA Health guidance warns that contact with contaminated water can lead to infections of the skin, ears, eyes or digestive system.

Shocking does not make the pool swimmable straight away either. SA Health advises not to use the pool until free chlorine falls back to 4 mg/L, which may mean leaving it overnight, and not to use it at all while the pump and filter are not working properly. A slightly green or hazy pool is in the same category: wait until it is clear, retested and in range. Keep children supervised and the pool gate shut throughout the treatment, as SA Health stresses for every home pool.

Pool chemical safety while you treat it

Green pool recovery uses more chemicals than usual, so handling them safely matters as much as the dose. HealthyWA, the WA Department of Health, warns that pool chemicals can react violently with household chemicals and with each other. WorkSafe WA lists liquid pool chlorine as incompatible with acids, including pH decreaser products.

• Never mix chemicals together.
• Add chemicals to water, never water to chemicals, and use a separate clean scoop or bucket for each product.
• Never let chlorine and acid come into contact; add them separately, following each label and Safety Data Sheet (SDS).
• Store chemicals away from children in a cool, dry, lockable place, as HealthyWA advises.
• If chemical gets in an eye, flood it with clean running water and get to a doctor or hospital immediately.

For poisoning advice, call the Poisons Information Centre on 13 11 26; in an emergency, call 000. If you are unsure about a product, ask before you pour: pool chemical supplies on the Sunshine Coast.

When to stop DIY and call a pool service

Call a pool service when the pool is heavily green with the floor hidden, when equipment is part of the problem, or when algae persists despite correct treatment (SA Health advises consulting your pool supplier). The water chemistry is often owner-safe; the boundary is equipment, electrics and the pool structure.

Owner-safe: testing, emptying baskets, brushing, adding chlorine, acid and clarifier exactly to the label, cleaning a cartridge or backwashing a sand filter, and checking the pump primes and the timer settings.

Call a pro: a pump that hums but will not start, trips the safety switch, leaks or overheats; a cracked filter or lid; or a salt cell that no longer produces chlorine. Any electrical work on a pump, chlorinator, heater or light must be done by a licensed electrician; do not open wiring or electrical housings yourself. Queensland’s Electrical Safety Office says to always get a licensed electrician to do any electrical work, and counts repairing electrical equipment as electrical work. After a flood, Queensland Government recovery advice adds that a licensed electrician must inspect affected pool electrical equipment and declare it fit before use. For pumps, see pool pump repairs.

Do not drain it yourself. Lo-Chlor’s light, medium and heavy algae treatments do not involve emptying the pool, and it suggests emptying and refilling only when phosphate is above 5.0 ppm. Emptying can also harm the structure: NSW Health’s flood advice warns that an empty pool, particularly a fibreglass one, may pop out of the ground, and Queensland Government flood-recovery advice says to ask your local pool shop when water can be pumped out without damaging the structure. To book a pool clean, contact our pool cleaning service, or find your nearest shop in Noosa, Maroochydore or Caloundra.

How to stop the pool going green again

Stopping a green pool coming back means keeping free chlorine in range every day, because algae spores are always arriving. Lo-Chlor notes spores are always present in the air, so prevention is a daily job rather than a monthly rescue.

1. Test pH and free chlorine at least daily in use, and more often in hot weather, as SA Health advises.
2. Keep free chlorine at or above the minimum in the required-levels table on your chlorine label (for a stabilised outdoor pool, 2.0 mg/L, or 3.0 mg/L once the water is above 26°C), and keep pH at 7.2 to 7.6.
3. Keep stabiliser between 30 and 50 mg/L so sunlight does not strip chlorine, without letting it build up.
4. Keep disinfecting when the pool is not in use; SA Health says pool water should be disinfected at all times.
5. Brush walls, steps and corners, and clean the baskets, lint pot and filter regularly.
6. Test and shock after heavy rain, storms or a pool party.
7. Use a cover when the pool is idle; SA Health says it keeps leaves and debris out. See is a pool cover worth it.

If keeping on top of it is the problem, a regular pool maintenance visit takes the testing and dosing off your list.