How to Fix a Green Pool in Auckland

Why does a pool turn green?

Algae is almost always the culprit. When your chlorine level drops — even briefly — algae can take hold surprisingly fast. Auckland's UV intensity is particularly unforgiving: on a hot summer day, chlorine can burn off faster than you'd expect, sometimes within just a few hours at peak UV.

Other common triggers include heavy rain (which dilutes your chemicals), being away on holiday and not having anyone check the pool, or simply a filter that's not running long enough each day. The water goes green because algae is literally growing in it — millions of single-celled organisms that thrive in warm, under-chlorinated water. They multiply quickly, and once they've got a foothold your pool can go from slightly hazy to full swamp-green in 24–48 hours.

The good news: algae isn't permanent. With the right approach and a bit of patience, you can turn things around.

Can you fix it yourself?

Honest answer: yes, for mild green — where you can still see the bottom of the pool. A light green pool with visible water underneath is usually fixable with the right chemicals in the right order over a couple of days.

But if the water is dark green or black — if you genuinely can't see the bottom at all — or if you've already tried to fix it and it came back, that's a different story. Getting the chemical sequence wrong can actually make things worse, and some algae strains are genuinely resistant to standard chlorine shock. In those cases, calling a professional is faster, cheaper overall, and a lot less frustrating than running through multiple failed attempts.

If you're in the mild-green camp and want to give it a go yourself, here's what to do.

Step-by-step: fixing a mildly green pool yourself

1 Test your water first

Before you add anything, test your water — at minimum pH and chlorine. You can buy a basic test kit from Bunnings or most pool shops for around $20–$30, or grab test strips if you want something quick and simple. You need to know where you're starting from before you start throwing chemicals in. Going in blind is one of the most common reasons DIY fixes don't work.

2 Adjust pH to 7.2–7.4

This step is critical and often skipped — and it's the reason a lot of people burn through two bags of shock with no result. Chlorine works dramatically more efficiently at the right pH. If your pH is too high (above 7.6), chlorine becomes far less effective — you can dump in a lot of shock and it barely touches the algae.

Use pH Down (sodium bisulphate) to lower it, or pH Up (sodium carbonate) to raise it. Get it into the 7.2–7.4 range before you move on. Give it 30–60 minutes after adding pH adjuster before you retest.

3 Shock the pool

"Shocking" means adding a large dose of chlorine — typically 3–5 times the normal maintenance amount — to overwhelm and kill the algae. Use granular chlorine (calcium hypochlorite or sodium dichloro) dissolved in a bucket of water first, then pour it evenly around the edges of the pool. Don't dump it all in one spot.

Do this at dusk or in the evening. UV light degrades chlorine quickly, so dosing at night gives it the best chance to work through the water before the sun starts breaking it down again.

4 Run the filter continuously for 24–48 hours

Don't turn the filter off. You need the water moving and the filter working hard to capture the dead algae particles as they're killed off. Check the filter pressure every 12 hours or so — if it rises significantly, backwash or clean the filter to keep your flow rate up. A clogged filter won't do much good, and it can also put strain on your pump.

5 Vacuum to waste

Once the water starts clearing — usually after 24–48 hours — you'll likely have a layer of dead algae sitting on the pool floor. This is actually a good sign: it means the shock is working. Now you need to vacuum it out on the "waste" setting (sometimes labelled "drain" on older equipment), which sends the water straight to drain rather than back through your filter.

This is important: if you vacuum to filter, you'll clog it with dead algae and the water will just recirculate the mess. You will lose some water this way, so top up your pool afterwards and recheck your chemical levels.

6 Retest and balance your water

Once the pool is clear, don't just call it done. Test again and balance your full chemistry: pH, free chlorine, total alkalinity, and calcium hardness. A balanced pool is a pool that stays clear — and it's also easier on your equipment and your skin. Aim for free chlorine at 1–3 ppm, pH at 7.2–7.4, total alkalinity at 80–120 ppm, and calcium hardness at 200–400 ppm.

When to call a professional

Some situations really do call for a pro. Don't spend days and dollars going around in circles — give us a call if:

• The pool is dark green or black — you can't see the bottom at all
• You've gone through the steps above and the pool is still green after 48–72 hours
• The pool cleared briefly but has turned green again within a week
• You don't have the right chemicals on hand and don't want to risk getting the sequence wrong
• You just don't want the hassle — and honestly, that's a completely valid reason

A severe algae bloom often requires multiple rounds of shock treatment, an algaecide, and sometimes a flocculent (a clarifying agent that causes suspended particles to clump together so they sink to the floor and can be vacuumed out). Getting that right takes experience, and doing it wrong just drags the process out.

Frequently asked questions