Understanding Why Your Pool Keeps Turning Green – Even with Chlorine

As a Kiwi homeowner, there’s nothing more frustrating than looking forward to a swim, only to find your pool has turned an unpleasant shade of green. You test the water, see the chlorine levels are fine, and yet the green persists. What’s going on?

Here in New Zealand, our climate can create the perfect conditions for algae growth, even when you’re adding chlorine. Let’s take a closer look at the possible reasons your pool keeps turning green and what you can do about it.

1. Is Your Chlorine Doing Its Job?

Although adding chlorine is essential for keeping your pool clean, it’s not always enough on its own. Chlorine only works effectively if other key factors are balanced.

• Free Chlorine vs. Combined Chlorine: If your free chlorine (the active part that fights bacteria and algae) is too low, it might not be powerful enough to kill off algae. You may have enough total chlorine, but a build-up of combined chlorine (chlorine that has already reacted with contaminants) can reduce the effectiveness of your pool sanitiser.
• Sunlight & Chlorine Burn-Off: New Zealand’s strong UV rays can cause chlorine to break down quickly. If you’re not using a stabiliser like cyanuric acid, your chlorine may not last long enough to be effective.

A great resource for understanding chlorine balance is Swimart NZ, which offers expert advice on pool maintenance.

2. Are Your Pool’s pH & Alkalinity Levels Balanced?

Your pool water’s pH and total alkalinity play a big role in whether your chlorine can do its job properly. In New Zealand, we often have issues with pH drifting up due to factors like frequent rain and minerals in the water.

• Ideal pH Range: Between 7.2 and 7.6 – anything higher can weaken your chlorine’s ability to fight algae.
• Total Alkalinity (TA): Aim for 80-120ppm to help stabilise pH and maintain chlorine efficiency.

For balancing products and testing kits, check out local suppliers like Para Rubber or Pool & Spa Warehouse NZ.

3. Phosphates & Nutrients: Are You Feeding the Algae?

Even if chlorine is present, algae can still thrive if there are excess nutrients in your pool. Phosphates from leaves, lawn fertiliser run-off, or rainwater can provide a food source for algae, encouraging green water.

• How to Fix It: Use a phosphate remover, regularly skim leaves from the surface, and consider covering your pool when not in use. Products like phosphate removers can be found at The Warehouse, which stocks some basic pool care essentials.

4. Is Your Filtration & Circulation Working Properly?

If your pool pump and filter aren’t operating efficiently, your chlorine can’t circulate properly, leaving stagnant areas for algae to form.

• Run your pump for at least 8 hours a day, especially in summer when algae thrive.
• Backwash and clean your filter regularly to ensure proper circulation.
• Check for dead spots where water isn’t moving properly – these areas are prime places for algae to grow.

For troubleshooting filtration issues or upgrading your system, Kiwi Pool & Spa has useful advice and equipment.

5. Heavy Rain & Weather Conditions

New Zealand’s unpredictable weather can impact your pool chemistry more than you realise.

• Heavy rain can dilute your chlorine levels, making it easier for algae to bloom.
• Winds bring additional debris and organic contaminants into the pool, further feeding algae.

After a storm, always test and rebalance your pool water. If needed, give it a shock treatment using a fast-acting chlorine product.

6. When to Use a Shock Treatment

If your pool is persistently green despite balancing chlorine and pH, it might be time for a shock dose. Super-chlorinating your pool with a high dose of chlorine (or a non-chlorine shock) is often the best way to kill off stubborn algae.

• Follow dosage recommendations from your pool chemical supplier.
• Run the pump continuously for at least 24 hours after shocking.
• Brush the walls and floor to help remove dead algae.

For shock treatments, Bunnings NZ has a selection of products you can check out here.

7. Preventative Maintenance – Keeping Your Pool Blue All Year

Once you’ve cleared up a green pool, keeping it well-maintained will help prevent future problems.

• Test your water at least once a week (more often in summer).
• Keep chlorine and pH levels in the correct range.
• Remove leaves and debris promptly.
• Keep your pump and filter running consistently.

For ongoing maintenance advice, Swimart NZ offers professional water testing services to ensure your pool stays balanced.

Final Thoughts

If your pool keeps turning green even with chlorine, it’s often due to imbalances in pH, alkalinity, or poor circulation. By maintaining proper water chemistry, keeping your pool clean, and ensuring your filtration system is working correctly, you can enjoy crystal-clear water all summer long.

Need help? Many local pool stores offer free or low-cost water testing—check with your nearest Swimart or Para Rubber to get professional advice tailored to your situation.

Now, time to get that pool looking as inviting as a classic Kiwi summer day! 🏊‍♂️🌞