If your pool water is starting to look green, it’s a clear sign something’s wrong. It might seem like a small change at first, but the sooner you act, the easier and cheaper it is to fix. Green water means algae are growing, and once that starts, the problem spreads fast.
As one of our Pool Techs, Rohan, likes to say, “a pool will tell you long before it goes green, you just have to know how to read the signs.” That’s the real secret to keeping your pool healthy. Understanding what those early signs mean can save you time, money, and a lot of frustration.
Below, we’ll go through the main reasons your pool can turn green, how to spot the warning signs, what to do if it happens, and how to stop it from coming back.
How do I know my pool could turn green?
Before the water goes bright green, there are smaller warning signs. These are easy to miss, but catching them early saves you time and money.
- The water looks dull or hazy – Clear water that starts to lose its sparkle is often the first stage. Tiny algae spores begin to grow, even if you can’t see them yet.
- You notice slippery pool walls or steps – A light film on surfaces means algae are starting to form. This is your first physical clue.
- Chlorine levels drop quickly – If your test shows low chlorine even after adding chemicals, it’s being used up fighting unseen growth.
- The pool smells stronger than usual – That heavy “chlorine” smell isn’t actually chlorine, it’s chloramines, a by-product of poor water balance and organic matter.
- You see green dust or patches forming – Once colour appears, even slightly, you’re already in the growth phase. The water will quickly move from pale green to dark green if untreated.
Why does a pool turn green?
There’s a myth that rain on its own turns a pool green. It doesn’t. What actually happens is a simple chain reaction that catches Sydney pool owners out all the time.
You get a run of heavy rain. The rain dilutes your chlorine and knocks your water balance out. Chlorine drops fast. The pool looks fine, so you leave it. Then the weather flips. Sydney gets four hot, sunny days in a row. Water temperature climbs over 25 degrees. You haven’t checked the pool in a week. The chlorine is now sitting too low to protect the water. Algae thrive in warm water, and the first spots appear on the shaded side of the pool, often the southern-facing walls.
This is the perfect setup for a green pool. It’s not dramatic weather. It’s normal Sydney summer conditions. Rain followed by heat. A few unchecked days. And the water shifts from clear to cloudy to green before you even notice.
There are actually many reason a pool can turn green but lets break it down. It’s a mix of water chemistry, filtration, and maintenance habits.Here are the main causes we see across Sydney’s Northern Beaches and Eastern Suburbs.
- Low chlorine levels – Chlorine kills algae. When levels drop too low, algae have the perfect chance to grow.
- Poor circulation or filtration – If your filter is clogged, your pump isn’t running long enough, or your return jets are blocked, water can stagnate and algae can multiply.
- Unbalanced water chemistry – When pH or alkalinity is off, chlorine becomes less effective. Even if you add the right amount, it won’t work properly.
- Heavy rain or debris – Storms wash in organic matter and phosphates, which act as algae food. Fallen leaves, dust, and pollen also feed growth.
- Faulty or old equipment – A worn-out filter, undersized pump, or malfunctioning chlorinator can all lead to green water over time.
I have a green pool, what can I do?
Once your pool is green, it’s tempting to dump in more chemicals, but that’s rarely the full solution. Acting quickly helps, but it’s important to do things in order.
- Test the water – Check chlorine, pH, and alkalinity. If chlorine is zero or very low, it confirms algae are active.
- Clean the pool – Remove all leaves and debris. Brush walls, steps, and corners to loosen algae before you add any treatments.
- Shock the pool – This involves adding a high dose of chlorine or an algaecide to kill off the growth. If done incorrectly, it can waste chemicals or damage the surface, so this step is best handled by a professional.
- Run the filter continuously – After shocking, your filter needs to run for at least 24–48 hours to clear dead algae from the water.
- Vacuum and backwash – Once the water begins to clear, vacuum the remaining debris and backwash your filter to flush it out.
If your pool stays green even after treatment, it’s a sign your filter media might be old or clogged. At The Pool Service Co., we often find the problem comes down to worn-out cartridges or sand filters that haven’t been changed in years.
How to treat a green pool professionally
Professional pool technicians have tools that make the process faster and safer. We can use digital testing to pinpoint what’s out of balance, apply the right shock dose, and ensure your filtration system is working at full strength. We also check your equipment for hidden issues that cause recurring algae problems, like broken chlorinators or undersized pumps.
A proper clean-up by a professional usually restores water clarity within a few days, depending on how severe the problem is.
How to prevent a green pool
Keeping your pool from turning green is easier than fixing it. Prevention is all about consistency.
- Maintain steady chlorine levels – Check your chlorine weekly and top it up as needed. Automated systems help but still need regular checks.
- Keep your filter clean – Backwash or replace cartridges as per manufacturer guidelines. A dirty filter can’t trap fine particles or algae spores.
- Balance the water regularly – Aim for pH between 7.2 and 7.6, and keep alkalinity in range. Balanced water keeps chlorine effective.
- Brush and skim weekly – Even clear water builds up film and dust. Brushing stops algae from settling. Focus on the southern-facing walls because they get less light and algae grow faster in shaded areas.
- Watch after storms – Heavy rain changes your water chemistry fast. Test and rebalance the day after big weather.
- Get professional servicing – A monthly or fortnightly visit from The Pool Service Co. ensures your system stays clean, balanced, and running efficiently year-round.
A green pool is always a warning sign. The quicker you act, the easier it is to bring it back. Whether it’s poor filtration, low chlorine, or a balance issue, catching the signs early saves you money and protects your swimming time.
Every pool behaves differently. Each one has its own patterns and quirks. Some respond fast to changes. Others take more work. This is why The Pool Service Co assigns dedicated pool techs to the same pools each week. They learn your pool, understand how it reacts, and know what it needs to stay clean and stable. Rohan describes pools as having their own personalities. Some are stubborn. Others are cheeky. A few are relaxed and easy. When you understand your pool, you know how to look after it.
If your pool water has started to change colour or lose clarity, contact The Pool Service Co today. We will test, treat, and restore your pool fast, so you can get back to swimming.
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