If you’re standing at the edge of your pool wondering why is my pool green, you’re definitely not the only one. I see this all the time, especially during a hot Kiwi summer run of sunny days, a bit of rain, or when the pool hasn’t had much attention for a week or two. One day it looks fine, the next it’s gone dull, cloudy, or full-on pea soup green.
The good news is this. A green pool is almost always fixable, and in most cases, you can sort it yourself with the right approach. You don’t need magic chemicals or guesswork. You just need to understand what’s actually happening in your water, and then deal with it properly.
I’ve been working with pools for years across Auckland, and I’ll walk you through exactly why pools turn green, what to do about it, and how to stop it happening again.
Let’s deal with the worry first
A green pool doesn’t automatically mean:
- Your pool is ruined
- You need to drain it
- You’ve done something terribly wrong
Most green pools come down to simple chemistry issues combined with algae growth. Algae love warm water, sunlight, and neglected or under-sanitised pools. Sound familiar? That’s basically a New Zealand summer in a nutshell.
Before we jump into fixing it, let’s talk about what’s actually causing the problem, because guessing is one of the biggest mistakes pool owners make.
What actually makes a pool turn green?
When someone asks me “why is my pool green?”, there are a few usual suspects. Sometimes it’s just one issue, but often it’s a combination.
Algae growth (the most common cause)
In New Zealand, algae is public enemy number one for pools. High UV levels, warm temperatures, and long daylight hours make it easy for algae to explode if chlorine drops even slightly too low.
Algae spores are always present. Wind, rain, swimmers, leaves, even birds introduce them into your pool. If your sanitizer isn’t strong enough, algae takes over fast.
Green water usually means:
- Free chlorine is too low
- Chlorine is being destroyed by UV
- Filtration is not keeping up
Low or ineffective chlorine
Sometimes people say, “But I’ve got chlorine in the pool.” That doesn’t always mean it’s working.
Chlorine becomes ineffective when:
- pH is too high
- Cyanuric acid is too high or too low
- There’s a heavy organic load, like leaves or pollen
- The chlorine demand is higher than usual
This is why testing matters, not guessing.
High pH levels
High pH makes chlorine lazy. When pH creeps above about 7.6, chlorine loses a lot of its killing power. You can have “normal” chlorine readings and still get algae if pH is out.
In NZ, high pH is common due to:
- Concrete and plaster pools
- Use of alkaline water supplies in some regions
- Overuse of baking soda or buffer
Poor filtration or circulation
Your filter does more than catch dirt. It removes dead algae and fine particles that cloud the water.
Common issues I see:
- Filter not running long enough
- Dirty sand or cartridge
- Blocked skimmer baskets
- Poor circulation in corners or steps
If water isn’t moving properly, algae finds quiet spots to grow.
Metals in the water (less common, but confusing)
Sometimes a pool looks green but it’s not algae at all. Metals like copper or iron can tint water green, especially after shocking.
This often happens when:
- Bore water or rural water supplies are used
- Copper-based algaecides are overused
- Old heat exchangers start breaking down
The water will usually be clear, not cloudy, if metals are the issue.
Before you fix anything, get the basics right
One of the biggest mistakes pool owners make is throwing products in without knowing what’s going on. That usually wastes money and makes the problem worse.
Before you touch the water, you need to test it properly.
What you need before you start
Gear and supplies you’ll want on hand
- A reliable pool test kit or test strips
- Liquid chlorine or calcium hypochlorite
- pH reducer (acid)
- Pool brush suitable for your surface
- Leaf net or scoop
- A clean, working filter
- Time and patience
If you’re unsure about your test results, most local pool shops in NZ will test a water sample for free or cheap. Just make sure you take the sample from elbow depth, not the surface.
Understanding your test results (this matters)
Here’s what I want you to focus on when your pool is green:
- Free chlorine: This is the active sanitizer. If it’s near zero, algae has likely taken hold.
- pH: Aim for around 7.2 to 7.4 when treating algae.
- Cyanuric acid (stabiliser): Helps protect chlorine from UV. Too much makes chlorine ineffective.
- Total alkalinity: Important, but secondary during a clean-up.
If you’d like a deeper explanation, this is covered in more detail in our guide on pool water testing and balancing.
Why guessing makes green pools worse
I see people do this all the time:
- Add algaecide first
- Throw in a few chlorine tablets
- Run the filter for a couple of hours
- Hope for the best
What usually happens is the pool goes cloudy, stays green, or gets worse.
Algae doesn’t die politely. When you kill it properly, it turns grey or white and needs to be filtered out. That process only works if chemistry, circulation, and filtration are all working together.
Next, I’ll show you exactly how to fix a green pool step by step, including how much chlorine to use, how long it takes, and what changes if your pool is light green versus swamp-level green.
How to fix a green pool properly (step by step)
Now that you understand why your pool is green, let’s get into the part you actually care about. Fixing it. This method works for the vast majority of residential pools in New Zealand, whether it’s lightly green or looks like a swamp.
The key is doing things in the right order. Skip steps or rush it, and you’ll be chasing your tail.
Step 1: Remove as much physical debris as possible
Before touching the chemistry, get rid of anything organic in the pool.
- Scoop out leaves, twigs, insects, and debris
- Empty skimmer baskets and pump baskets
- If the pool is very green, you may not see the bottom, but still remove what you can
Organic debris eats chlorine. The more you remove now, the less chlorine you’ll need later.
Step 2: Brush the entire pool
This step is often skipped, and it’s a big reason green pools don’t clear.
Brush:
- Walls
- Floor
- Steps
- Corners
- Behind ladders and lights
Brushing breaks algae off surfaces and suspends it in the water so chlorine can actually kill it. I recommend brushing before adding chlorine and again daily until the pool clears.
Step 3: Balance pH first (this is critical)
Before adding large amounts of chlorine, adjust your pH.
- Target pH: 7.2 to 7.4
- Use pH reducer (acid) as needed
- Circulate for at least 30 to 60 minutes
- Retest to confirm
At this pH range, chlorine is far more effective. If you skip this, you’ll use more chlorine for worse results.
Step 4: Shock the pool with chlorine
This is where most of the algae is killed.
For a green pool, I recommend using:
- Liquid chlorine (sodium hypochlorite), or
- Calcium hypochlorite granules
Avoid tablets for shocking. They dissolve too slowly and don’t raise chlorine fast enough.
How much chlorine do you need?
As a general guide:
- Light green water: 5 to 10 ppm free chlorine
- Medium green: 10 to 15 ppm
- Dark green or swampy: 15 to 20 ppm
For a typical 40,000 litre pool, that can mean several litres of liquid chlorine. This is normal and expected.
Add chlorine in the evening if possible. This reduces UV burn-off and gives chlorine more time to work overnight.
Step 5: Run the filter continuously
Once chlorine is in, your job isn’t done.
- Run the pump 24 hours a day during the clean-up
- Backwash sand or DE filters as pressure rises
- Clean cartridge filters daily if needed
Dead algae will cloud the water before it clears. This is a good sign, not a bad one.
Step 6: Brush and test daily
Each day:
- Brush the pool once or twice
- Test chlorine and pH
- Top up chlorine if it drops too quickly
If chlorine is disappearing within hours, algae is still alive and chlorine demand is high. Keep levels elevated until they hold overnight.
Step 7: Clarify and polish the water (optional)
Once the water turns blue but cloudy, a clarifier can help.
Use:
- A liquid clarifier designed for pools
- Follow dosage exactly
- Don’t overdose
Avoid flocculants unless you know how to vacuum to waste. They work, but they’re easy to misuse and can create more problems if done wrong. This is covered in more detail in our guide on flocculants for pools.
How long does it take to clear a green pool?
This depends on how bad it is and how consistent you are.
- Light green pool: 2 to 3 days
- Medium green: 3 to 5 days
- Dark green or neglected pool: 5 to 7 days, sometimes longer
If nothing changes after several days, something is being missed. Usually pH, filtration, or stabiliser levels.
Common mistakes that make green pools worse
I see these all the time, even with well-meaning pool owners.
Adding algaecide instead of chlorine
Algaecides are preventative, not cures. They don’t replace proper chlorine levels and can add unnecessary chemicals to the water.
Using tablets as the main fix
Chlorine tablets are slow-release. They’re great for maintenance, not for killing active algae.
Not running the filter long enough
Six or eight hours isn’t enough during a clean-up. Filtration is half the job.
Ignoring stabiliser levels
Too much cyanuric acid locks up chlorine. If your stabiliser is high, shocking becomes less effective. This is covered in more detail in our guide on how to lower cyanuric acid in a pool.
Giving up too early
A green pool doesn’t clear instantly. If chlorine drops overnight, algae is still present. Stay the course.
What to avoid doing
- Don’t mix chemicals directly together
- Don’t add acid and chlorine at the same time
- Don’t swim until chlorine levels return to normal
- Don’t drain the pool unless advised by a professional
Draining can damage some pools and is rarely necessary for algae alone.
How to stop your pool turning green again
Once your pool is clear, keeping it that way is much easier.
Maintain consistent chlorine levels
In summer, I recommend:
- Testing 2 to 3 times per week
- Keeping free chlorine within the recommended range
- Adjusting for heatwaves, parties, and heavy rain
Run the filter long enough
A good rule of thumb in NZ summer conditions is:
- 8 to 12 hours per day minimum
- More during heatwaves or high use
Brush regularly
Even when the pool looks clean, brushing once a week helps prevent algae from getting a foothold.
Keep stabiliser in check
Chlorine needs protection from UV, but too much stabiliser causes problems. Regular testing avoids surprises.
Stay ahead after storms
Heavy rain introduces contaminants and dilutes chemicals. Always test after a storm.
When to call a professional
Sometimes DIY reaches its limit. I recommend getting help if:
- The pool stays green after a week of proper treatment
- Metals are suspected
- You can’t maintain chlorine overnight
- The filter isn’t performing correctly
A professional can quickly diagnose issues that aren’t obvious from test strips alone.
Frequently Asked Questions
1. Is it safe to swim in a green pool?
No. Green water usually means algae and low sanitation. Wait until the pool is clear and chlorine levels are back in range.
2. Can I fix a green pool without chlorine?
Realistically, no. Chlorine is the most effective and affordable sanitizer for algae in NZ pools.
3. Why does my pool turn green after rain?
Rain adds contaminants and can dilute chlorine, especially during warm weather.
4. Should I drain my green pool?
Almost never. Draining is usually unnecessary and can damage the pool structure.
5. Why is my pool green but clear?
This often points to metals in the water rather than algae.
Final thoughts
If you’ve been asking why is my pool green, the answer is usually simple. Algae took advantage of a moment where chlorine, balance, or filtration slipped.
With the right steps, patience, and a bit of consistency, you can bring even a badly green pool back to clear blue. I’ve seen far worse than most backyard pools recover just fine.
Stay on top of testing, keep your filter working hard, and your pool will stay swimmable all summer long.