Why Is My Pool Turning Green?

Why Does a Pool Turn Green?

Pool water turns green because algae — microscopic aquatic plants — have taken hold. Algae spores are always present in your pool (they blow in on the wind, arrive on swimmers' bodies, and come in with your water supply). Under normal conditions, chlorine kills them faster than they can reproduce.

Your pool turns green when that balance tips — when chlorine levels drop too low or algae grow too fast for your sanitiser to keep up. This happens most commonly because of:

• Low or zero chlorine — skipping treatments, especially in summer
• High pH — even adequate chlorine becomes largely ineffective above pH 7.8
• Heavy use or rain — dilutes chemicals and adds organic matter (food for algae)
• Poor circulation — algae flourishes in stagnant water; a broken pump speeds this up
• High temperatures — algae reproduces exponentially faster in warm water. At 30°C+ (standard Brisbane summer), it can double every few hours
• Low cyanuric acid (CYA) — CYA protects chlorine from being destroyed by UV; without it, outdoor chlorine evaporates quickly

How Bad Is It? Understanding the Severity

Before you treat, assess how green your pool actually is. The treatment intensity changes based on severity:

Appearance	Severity	Treatment Needed
Light green tint, can see bottom	Mild	Shock + brush + filter
Green water, bottom barely visible	Moderate	Heavy shock + algaecide + filter
Dark green, cannot see bottom at all	Severe	Multi-day treatment + possible drain
Black or dark brown water	Extreme	Drain, acid wash, refill — call a pro

The Step-by-Step Green Pool Fix

Step 1: Test Your Water First

Before adding anything, test your pH and chlorine. This tells you how much work needs to be done. A high pH makes shock treatment much less effective — fix the pH first.

• If pH is above 7.6 — add acid to bring it down to 7.2–7.4 before proceeding
• If pH is already in range — proceed to shocking

Step 2: Brush the Entire Pool

Use a pool brush on a telescopic pole to scrub every surface — walls, floor, steps, corners. Algae clings to surfaces and must be dislodged so the chemical treatment can reach it. Don't skip this step.

Step 3: Shock the Pool

Pool shock (calcium hypochlorite or liquid chlorine) is concentrated chlorine. For a green pool, you're going to use 3–5x the normal dose. This is called "super-chlorinating" or "shocking."

As a guide for a standard 50,000 litre pool:

• Mild green: 1–2kg of granular shock, or 10–15L of liquid chlorine
• Moderate green: 2–3kg of granular shock, or 15–20L of liquid chlorine
• Dark/severe green: 3–5kg of granular shock, or 20–30L of liquid chlorine

Add the shock at dusk or night — UV from sunlight destroys chlorine rapidly. Dissolve granular shock in a bucket of pool water first, then pour it around the pool perimeter. Run the pump on full.

Step 4: Add an Algaecide

After shocking, add a pool algaecide (follow the bottle's dosage for your pool volume). Algaecide works differently to chlorine — it attacks the algae cell structure. Used together they're more effective than either alone.

Step 5: Run the Filter Continuously

Run your filter 24 hours a day until the water is clear. As algae dies off it turns grey/white and gets captured by the filter. Clean or backwash your filter every 4–6 hours — it will clog up much faster than usual. For a moderate green pool, expect 24–72 hours of continuous filtration.

Step 6: Vacuum Dead Algae to Waste

Once the water starts clearing (grey/murky rather than green), vacuum the dead algae off the floor. Set your multiport valve to "Waste" not "Filter" — this bypasses the filter and pushes water out of the pool. This is important because vacuuming dead algae through the filter just overwhelms it.

You'll need to top the pool up with fresh water as you vacuum to waste.

Step 7: Retest and Balance Chemistry

Once the water is clear, do a full chemistry test. After all that shock treatment, your chemistry will be well out of balance. Test and adjust:

• pH (target 7.2–7.6)
• Chlorine (let it drop to 1–3ppm before swimming)
• Total Alkalinity (80–120ppm)
• Cyanuric Acid (30–50ppm)

How to Stop Your Pool Turning Green Again

A green pool is a symptom of neglected chemistry. Here's what to do going forward:

• Test every week in summer, fortnightly in winter
• Keep chlorine above 1ppm at all times — don't let it hit zero
• Maintain cyanuric acid at 30–50ppm — it protects outdoor chlorine from UV destruction
• Run the pump at least 8 hours per day in summer — circulation is key
• Brush walls weekly — algae starts on surfaces before it clouds the water
• Add a preventative algaecide monthly — especially through summer