A green pool doesn’t happen overnight. It’s usually the result of one (or more) of five things going wrong. Here’s how to diagnose the problem — and what the fix looks like.

1. Insufficient chlorine

The cause: Chlorine kills algae. If your level drops below 1ppm, algae gets a foothold.

The fix: Shock the pool with 5–10x the normal chlorine dose. Run the filter 24/7 for 2–3 days. Test daily.

Prevention: Maintain 2–4ppm free chlorine at all times. In Queensland’s UV, that means checking twice weekly in summer.

2. Poor filtration

The cause: Your filter is clogged, undersized, or not running long enough.

The fix: Backwash sand filters. Clean cartridge filters. Check your pump’s flow rate — if it’s dropped 20% from baseline, something’s blocked.

Prevention: Run your pump 8–10 hours daily in summer. Backwash every 4–6 weeks.

3. Imbalanced pH

The cause: pH above 7.8 makes chlorine ineffective. pH below 7.2 corrodes equipment.

The fix: Test and adjust pH before adding any chemicals. Use soda ash to raise, muriatic acid to lower.

Prevention: Test weekly. Adjust promptly.

4. Phosphates

The cause: Phosphates are algae food. They enter via leaves, fertiliser runoff, and even some pool chemicals.

The fix: Use a phosphate remover. Then shock and filter intensively.

Prevention: Skim daily. Trim overhanging trees. Use a phosphate test monthly.

5. Cyanuric acid overload

The cause: Too much stabiliser (CYA) locks up your chlorine. It can’t do its job.

The fix: There’s no chemical remover. You must partially drain and refill.

Prevention: Keep CYA at 30–50ppm. Don’t use stabilised chlorine tablets exclusively.

When to call a pro

If your pool is dark green, smells, or has visible algae on walls — it’s past DIY. A professional recovery service will:

Test all 6 chemical parameters
Apply the correct shock treatment
Run a flocculant if needed
Monitor daily until clear
Fix the root cause so it doesn’t repeat

Book a green pool recovery →

Why Is My Pool Green? The 5 Most Common Causes (And How to Fix Them)