Why Did My Pool Turn Green? Common Causes and How to Fix It

You walk into your backyard expecting to see clear blue water, and instead you are staring at something that looks more like a swamp than a swimming pool. It is a sinking feeling that Perth pool owners know all too well, and it happens more often than you might think.

A green pool is not just unsightly. It is a health hazard, a breeding ground for mosquitoes, and the longer you leave it, the more expensive it becomes to fix. Understanding why your pool turned green is the first step to getting it back to normal and preventing it from happening again.

The Science Behind a Green Pool

The green colour in your pool is almost always caused by algae. Specifically, it is usually green algae (Chlorophyta), which is the most common type found in Australian swimming pools. Algae are microscopic plant organisms that are present in every pool at all times as spores. Under the right conditions, they multiply rapidly and turn your water from clear to green in as little as 24 to 48 hours.

The conditions that trigger algae growth are:

• Low or no chlorine: Chlorine is your pool’s primary defence against algae. When levels drop below 1 ppm, algae can take hold.
• Warm water: Perth’s warm climate provides ideal temperatures for algae growth, especially from October through April.
• Sunlight: UV light fuels photosynthesis in algae cells. Perth’s intense sunshine accelerates growth dramatically.
• Nutrients: Leaves, dirt, pollen, and other organic matter provide food for algae.

Common Causes of Green Pools in Perth

1. Chlorine Failure

This is the most common cause by far. Chlorine can fail for several reasons:

Salt chlorinator malfunction. Most Perth pools use salt chlorination. If your chlorinator cell is scaled, worn, or the control unit has a fault, chlorine production drops or stops entirely. Many pool owners do not realise their chlorinator has failed until the pool is already green.

Over-stabilisation. Cyanuric acid (stabiliser) protects chlorine from UV degradation. But when levels exceed 80 to 100 ppm, it locks up chlorine so effectively that the chlorine cannot kill algae. This is extremely common in Perth pools because stabiliser accumulates over time and does not break down naturally.

Insufficient chlorine for the conditions. A chlorine level that is adequate in winter may be completely insufficient in Perth’s summer heat. UV radiation, high temperatures, and heavy swimmer loads all consume chlorine faster.

2. Pump or Filter Failure

Your pump and filter work together to circulate and clean your pool water. When either fails, water becomes stagnant, chlorine distribution is uneven, and debris accumulates, all perfect conditions for algae.

Common pump issues include tripped circuit breakers, blocked impellers, and failed capacitors. Filter problems include channelling in sand filters, torn cartridge elements, and clogged filter media.

If your pump has not been running for even a few days during warm weather, a green pool is almost guaranteed.

3. Inadequate Pump Run Time

Even a functioning pump and chlorinator cannot keep up if they are not running long enough. In Perth’s summer, your pump should run 8 to 10 hours per day. We regularly see green pools where the owner has their pump on a timer set for just 3 to 4 hours, which is simply not enough during warm months.

4. Heavy Organic Load

Perth suburbs with mature trees are particularly prone to this problem. Areas like Mount Claremont, Dalkeith, Cottesloe, and Peppermint Grove have beautiful established gardens, but the leaf litter, pollen, and organic matter that falls into pools provides a constant food source for algae.

After storms, which are common in Perth from late autumn through winter, the volume of debris can overwhelm a pool’s filtration system in a single day.

5. Neglect During Holidays or Winter

Going on holiday without arranging pool care is a common trigger. Even two weeks without maintenance during summer can result in a full green pool. Similarly, many Perth pool owners reduce or stop pool maintenance during winter, assuming cooler temperatures will keep algae at bay. While growth is slower in winter, Perth’s mild conditions still support algae, especially during warm spells.

How to Fix a Green Pool

The recovery process depends on how green your pool is. Here is a general guide:

Mild Green (Can See the Bottom)

If you can still see the pool floor, the algae bloom is in its early stages and can usually be treated without draining.

1. Test and adjust pH to 7.2 (slightly lower than normal to maximise chlorine effectiveness)
2. Shock the pool with liquid chlorine (sodium hypochlorite) to raise free chlorine to 10 to 15 ppm
3. Run the pump continuously for 24 to 48 hours
4. Brush all surfaces to break up algae colonies
5. Backwash or clean the filter frequently during recovery
6. Vacuum dead algae from the pool floor once the water clears

Moderate Green (Cannot See the Bottom)

When visibility is limited, the algae population is significant and recovery takes longer.

1. Remove as much debris as possible manually
2. Test stabiliser levels. If above 80 ppm, you may need a partial drain before treatment will be effective
3. Adjust pH to 7.0 to 7.2
4. Triple shock the pool and maintain elevated chlorine levels for 48 to 72 hours
5. Run the pump 24 hours a day
6. Clean the filter multiple times during recovery
7. Add a quality algaecide as a backup treatment

Severe Green (Opaque, Swamp-Like)

If the water is completely opaque and has a strong odour, the pool may need a full drain and clean. This involves:

1. Draining the pool completely (requires care to avoid hydrostatic pressure damage)
2. High-pressure cleaning of all surfaces
3. Acid washing if surfaces are stained
4. Refilling and chemical balancing from scratch
5. Equipment inspection and repair

A severe green pool recovery is best handled by professionals. The chemicals required are hazardous, the volumes needed are large, and incorrect treatment can make the situation worse and more expensive.

When to Call a Professional

While a mild green pool can sometimes be managed by an experienced pool owner, we recommend calling a professional if:

• You cannot see the bottom of the pool
• You are unsure of your stabiliser level
• Your pump or chlorinator may not be working correctly
• The pool has been green for more than a week
• You have tried shocking and it has not improved within 48 hours

Our green pool recovery service includes a full water analysis, equipment check, and a treatment plan tailored to your pool’s specific condition. We handle everything from mild algae blooms to severely neglected pools that need a complete drain and clean.

Pool Cleaning Perth specialises in green pool recovery across the Perth metropolitan area. If your pool has gone green, do not wait for it to get worse. Call us on 08 7943 6637 for a same-day assessment and we will have your pool back to crystal clear as quickly as possible.