How to Fix a Green Pool: A Step-by-Step Guide for Cape Town Owners

By Aquatic Pools
June 2, 2026

You wake up after a warm, rainy Cape Town spell and your pool has turned green overnight. It’s frustrating — but a green pool is almost always fixable at home with the right steps in the right order. Here’s exactly how to clear it.

Why do pools turn green in Cape Town?

Green water means algae, and algae blooms when sanitiser drops and conditions are warm:

Low or no chlorine (the number-one cause)
Hot weather and strong sun burning off chlorine
Rain diluting and unbalancing the water
Poor circulation or a dirty/blocked filter
High phosphates feeding algae

How to fix a green pool, step by step

Remove debris. Net out leaves and twigs — they feed algae.
Brush the pool. Brush the walls and floor to lift algae off the surface (be gentle on fibreglass — use a soft brush).
Test and balance the water. Aim for pH 7.2–7.6 and total alkalinity 80–120 ppm. Chlorine works far better when these are right.
Shock the pool. Add a strong dose of chlorine (pool shock) late afternoon, following the dosage for your pool’s litres. Run the pump while dosing.
Run the filter 24 hours. Circulation is what actually clears the water — keep the pump running continuously.
Vacuum and add a clarifier/floc if the water is cloudy after shocking, then vacuum the settled debris to waste.
Backwash the filter as it loads up with dead algae.
Retest after 24 hours and re-shock if needed.

A mildly green pool can clear in 1–2 days; a dark-green “swamp” can take 4–7 days of repeated shocking and filtering.

When to call a professional

Call in help if the pool stays green after repeated shocking, if the water is so dark you can’t see the floor, or if the green won’t shift off the surface — which can signal algae embedded in a porous or failing marbelite finish. For year-round prevention, follow our Cape Town pool maintenance calendar.

Does a green pool damage the surface?

Persistent algae can stain and degrade porous marbelite, and black algae roots into the surface (see our guide on black algae and pool stains). If your pool greens up constantly and the surface is rough or stained, resurfacing or a fibreglass lining makes it far easier to keep clear.

How to stop your pool going green again

Keep chlorine in range, especially in summer
Run the filter long enough each day (longer in summer)
Backwash and clean the filter regularly
Shock after heavy rain or heavy use
Consider whether a salt-water system suits your pool

How much chlorine (shock) does a green pool need?

Dosage depends on how green the water is and your pool’s volume. As a rough guide, a mildly green pool needs a double dose of shock, while a dark “swamp” can need three to four times the normal dose, repeated daily until it clears. Always work to your product’s instructions for your pool’s litres, dose in the late afternoon, and keep the pump running to circulate it.

Fibreglass pools clear faster

Algae clings to porous marbelite, so green water on an old surface can be stubborn and keep returning. A non-porous fibreglass surface gives algae nothing to grip — green episodes clear faster and happen less often.

Frequently asked questions

Should I drain a green pool to clean it?
Almost never — draining risks the pool lifting out of the ground and is usually unnecessary. Clear it chemically with the pool full. See our guide on draining a pool in Cape Town.

How often should I shock my pool?
Every 1–2 weeks in summer, and after heavy rain, a pool party, or any time chlorine drops and the water dulls.

How long does it take to clear a green pool?
A light green pool clears in 1–2 days; a very dark pool can take up to a week of shocking, filtering and vacuuming.

Why is my pool still green after shocking?
Usually low free chlorine, unbalanced pH, a dirty filter, or dissolved metals. Balance the water, clean the filter, keep the pump running and re-shock.

Can I swim in a green pool?
No — algae and the bacteria that come with it are a health risk. Clear and balance the water first.