Why Your Pool is Green After Rain (And How to Fix It Fast)

Woke up to a green pool after a Cape Town storm? You are not alone, this happens to thousands of pool owners every winter. The good news: it is almost always fixable in 24 to 48 hours if you act fast. Here is exactly what to do.

Why rain turns your pool green

Rain itself is not green, but it is acidic and full of pollen, dust, leaves and other organic material. When a big storm hits a pool, three things happen at once:

Your chlorine gets diluted by the volume of rainwater added
Your pH drops because rainwater is slightly acidic
Huge amounts of organic material wash into the pool

The result: bacteria and algae spores that were dormant in the water now have low chlorine, the wrong pH, and plenty of food. They multiply rapidly, and within 12 to 24 hours your pool can go from clear to swamp green.

The fast fix

Step 1: Brush and skim (15 minutes)

Remove leaves and debris first. Brush the walls and floor to knock loose algae into suspension where the filter can grab it. This is not optional, brushing makes the rest of the treatment work properly.

Step 2: Test and balance pH (30 minutes)

Use a reliable test kit or take a sample to a pool shop. You want pH between 7.2 and 7.6. If pH is too low (common after rain), add sodium carbonate (pH up). Wait 30 minutes after adding any chemical before doing the next step.

Step 3: Shock the pool

You need to add a high dose of chlorine, far more than your normal weekly maintenance level. For a green pool, that usually means 3 to 5 times your normal chlorine dose. Add it in the evening (UV breaks chlorine down during the day).

If your pool is very green, you may need a flocculant or algaecide alongside the chlorine. A good pool shop or service technician will get the right combination for your situation.

Step 4: Run the pump continuously

For the next 24 to 48 hours, keep the pump running non-stop. Backwash the filter every 8 to 12 hours, the dead algae has to go somewhere, and it will clog your filter fast.

Step 5: Wait, retest, vacuum

By 24 to 48 hours, the water should be cloudy white or pale grey, not green. That is dead algae. Once the cloudiness settles to the bottom (overnight is usually enough), vacuum the floor manually. Do not just rely on the filter, you want to remove the dead algae from the pool entirely.

When to call a professional

Most green pools clear in 24 to 48 hours with the steps above. Call us if:

Your pool is dark green or has been green for over a week
The water is green and cloudy with no clearing after shock treatment
You have black or yellow algae (these are different and need different treatment)
Your pH or chemistry will not stabilise no matter what you add
You are seeing visible staining of the pool walls or floor

The longer a pool stays green, the harder it is to fix and the more chemicals you waste. Act in the first 48 hours and the cost is minimal. Wait a week and you may need a complete drain and refill.

Preventing it next time

The best treatment is not having to treat in the first place.

Keep your chlorine slightly higher in winter (most owners do the opposite)
Use a stabiliser to protect your chlorine from being burned off too fast
Cover the pool if a major storm is forecast
Test water within 12 hours of any heavy rain
Consider a robotic cleaner that runs daily, even in winter

Our weekly maintenance clients almost never see a green pool, because the chemistry is monitored continuously and adjusted before problems start. If a green pool every winter has become routine, it might be cheaper to put it on a service plan than to keep buying emergency chemicals.