Why Is My Pool Water Green? How to Fix It Fast

Green pool water means one thing: algae. It grows when the chlorine drops too low to kill it, usually after hot weather, heavy rain or a busy weekend of swimming. The good news is it is easy to fix, and you can start today. Follow these steps in order.

Quick answer

Your pool is green because algae has taken over. To fix it: test the water, correct the pH, brush the walls, add a big dose of shock at dusk, then run the filter non-stop until it clears. Most pools go from green to blue in 2 to 4 days.

Why did it happen?

Algae are tiny plants. They are always trying to grow in your pool, and chlorine is what stops them. When the chlorine drops, even for a day or two in warm weather, the algae bloom and the water turns green fast. It is very common, and it just means the chlorine ran low.

How to fix a green pool, step by step

You will need: a test kit, a pool brush, shock (chlorine), and pH adjuster. Our pool starter kit has the chlorine, pH plus, pH minus and algaecide in one box.

Step 1: Test the water

Use a test kit or strip to check the pH and chlorine. This tells you what the water needs. Do this first, always. If you do not have test strips you can always purchase them here

Step 2: Fix the pH

Get the pH to between 7.2 and 7.4. This matters, because if the pH is too high the chlorine cannot do its job. Add pH minus to bring it down, or pH plus to bring it up. Wait 20 minutes.

Step 3: Brush the walls and floor

Scrub every wall, step and corner with a pool brush. Algae clings on tight, and brushing knocks it loose so the chlorine can kill it. Do not skip this. It can halve the time it takes to clear.

If you want a hands free alternative to cleaning your pool consider getting an automatic pool cleaner

Step 4: Add shock, at dusk

Shock is a big dose of chlorine. Add it in the evening, not in daytime, because sunlight burns off chlorine before it can work. Pre-mix the granules in a bucket of water first, then pour it around the edge of the pool with the pump running. Follow the dose on the pack. Browse pool shock and chlorine here.

Safety: keep everyone out of the pool until the chlorine drops back below 5. Never mix chemicals together. Always add chemical to water, never water to chemical.

Step 5: Run the filter non-stop

Leave the pump and filter running 24 hours a day until the water clears. This pulls the dead algae out. Clean or backwash the filter when it gets dirty. The water will turn from green to cloudy grey or milky blue. That cloudiness is dead algae. It means it is working.

Step 6: Brush again, and retest

After a day, brush again and test the water. Still green? Add another dose of shock. Clearing up, but cloudy? Keep the filter running and it will settle. Once the chlorine is back to normal, add an algaecide to finish the job.

How to stop it coming back

Clearing a green pool is hard work. Keeping it clear is easy. Do these five things and it will not happen again.

• Keep chlorine at 1 to 3 at all times. This is the single most important thing. Low chlorine is what let the algae in.
• Add an algaecide every week as a backup shield. Our Algae Controller is copper-free and dosed weekly, or Blue Horizons Summerclear is a long-life option dosed every three months.
• Run the filter 8 to 12 hours a day in summer. Still water is where algae grows.
• Brush and skim weekly, even when it looks clean.
• Test the water twice a week in warm weather, so you catch low chlorine before the pool goes green.

If the green keeps coming back, our Ultimate Algicide is a stronger dual-action option that both cures and prevents.

FAQ

How long does it take to clear a green pool? Most pools clear in 2 to 4 days. A light green pool can clear in 24 to 48 hours. A very dark, swampy pool can take up to a week of shocking and filtering.

Can I swim in a green pool? No. Green water carries algae and bacteria that can cause ear, skin and eye infections. Wait until the water is clear and the chlorine has dropped back below 5 before swimming.

Why did my pool go green overnight? Because the chlorine ran out. In hot weather or after heavy rain, algae can bloom in less than a day once chlorine drops too low to stop it.

Do I need to drain my pool if it is green? Almost never. Green water clears with shock and filtering. Draining is only needed in extreme cases, usually where the stabiliser level is far too high.

Get what you need

Everything to clear a green pool is in our pool chemicals range, or start with the all-in-one pool starter kit. New to pool care? Our beginner’s pool guide walks you through the basics. Not sure what your water needs? Send us a photo and your test readings, and we will tell you exactly what to add.