Why Your Pool Is Turning Green and How to Fix It

The green pool problem

You look out and the water has turned green. It is one of the most common pool problems in South Florida, where heat and humidity make algae easy to grow. The good news is that a green pool is usually fixable, often within a few days. This guide is general advice. Chemical amounts depend on your pool's size and condition, so follow product labels and ask a professional if you are unsure.

What causes it

Green water is almost always algae. Algae thrive in warm, still water without enough sanitizer. Common causes include:

• Low chlorine: The most common cause. Once free chlorine runs low, algae take hold quickly.
• Poor circulation: Dead spots let algae start.
• Heat: Warm water grows algae faster and breaks down chlorine faster.
• Too much stabilizer (CYA): Very high cyanuric acid makes chlorine less effective, even when a test shows a normal reading.
• Phosphates: Debris, fertilizer runoff and landscaping materials bring in algae food.
• Weak filtration: Not enough pump run time or a dirty filter.
• Heavy rain: Rain dilutes chlorine and brings in contaminants.

Types of algae

Green algae

The most common. It shows up as haze, floating patches or a green film. It is the easiest to treat.

Yellow (mustard) algae

A yellow-brown film, usually in shaded spots. It resists chlorine better and needs a stronger treatment.

Black algae

Dark spots rooted in the surface, especially plaster. It is the hardest to remove and usually needs aggressive brushing and a professional treatment plan.

How to clear a green pool

1. Remove large debris

Use a net. Do not run large debris through the pump and filter.

2. Test the water

Check free chlorine, pH, alkalinity, stabilizer and, if you can, phosphates.

3. Balance pH first

Chlorine works better at a lower, balanced pH. If pH is high, bring it down before you shock.

4. Shock the pool

Raise chlorine to a high shock level using liquid chlorine or granular shock, following the product's directions for your pool volume. A very green pool takes a lot of chlorine. Algae use it up quickly.

5. Brush everything

Brush walls, floor and steps to break up algae so chlorine can reach it. Use a brush that suits your finish. Plaster pools usually take a stiff brush, but pebble and some finishes do not, so check first.

6. Run the pump continuously

Run it around the clock until the water clears. That can take a few days.

7. Clean the filter often

Dead algae clog filters fast. Clean or backwash daily during recovery.

8. Keep testing

Test chlorine at least twice a day and add more as needed until the water is clear and holding chlorine overnight.

Preventing a repeat

• Keep free chlorine in the range your pool professional recommends and test weekly or more.
• Keep stabilizer in range. Too little wastes chlorine and too much weakens it.
• Run the pump long enough, especially in summer.
• Brush weekly, especially shaded areas and corners.
• Clean the filter on schedule.
• After heavy rain, test and adjust right away.
• Consider an algaecide or phosphate remover during summer.

Our pool maintenance guide lays out a weekly routine.

When to call a professional

Call for help if the pool is dark green or black, shocking does not clear it, black algae keeps coming back, or the pump and filter are struggling. A rough or porous surface can also hold algae that chemicals cannot reach. If your finish feels like sandpaper, pool resurfacing may be the real fix. EDX also handles pool equipment repairs when circulation or filtration is the issue. Text us photos for a first look.