What Causes My Pool Water to Turn Green Overnight?

Rapid algae growth due to low chlorine levels or poor circulation can cause green water. Shock the pool immediately and scrub surfaces.

Common Causes of Green Pool Water

Green water is primarily caused by algae growth, but other issues can contribute as well. The most common causes include:

• Algae growth: The leading culprit for green water is algae. Algae thrive in warm water, especially when the pool’s chlorine levels are too low to keep them in check. Algae reproduce quickly, turning your pool water green within hours.

• Chlorine imbalance: Insufficient chlorine means that algae spores aren’t getting killed off, leading to rapid algae growth. If your chlorine level is consistently low, you may notice the water changing color quickly.

• High pH and alkalinity: High pH levels, above 7.8, and elevated alkalinity can reduce the effectiveness of chlorine. When the chlorine cannot do its job properly, algae and other contaminants flourish, turning the water green.

• Metals in the water: Copper, iron, and other metals can discolor your pool water. High metal content can occur from metal pool equipment or from the water source. These metals often cause greenish hues in the water.

How to Fix Green Pool Water

If you find your pool water has turned green overnight, don’t worry! this problem can be fixed with a few simple steps. Follow these directions to restore your pool water to a crystal-clear state:

1. Test your pool’s water

Before addressing the issue, it’s important to measure your water’s chemical balance. Use a pool test kit to measure key levels:

• Chlorine levels: Ensure the chlorine level is between 1-3 ppm.

• pH level: The pH should be between 7.2 and 7.8.

• Alkalinity: Check that alkalinity is between 80-120 ppm.

• Calcium hardness: It should be between 200-400 ppm to avoid scaling.

2. Shock the pool

Shocking your pool with a high dose of chlorine is a key step to kill algae and bacteria. Here’s how to shock the pool:

• When to shock: It’s best to shock the pool in the evening or at night, as the sun can cause chlorine to dissipate too quickly.

• How much shock to use: Always follow the instructions on the shock treatment for the appropriate dosage, based on your pool size.

3. Run the pool filter

After shocking the pool, turn the filter on and run it for 24-48 hours to help clear up the algae and contaminants. Make sure to clean or backwash the filter regularly to prevent clogging.

4. Brush the pool surfaces

Use a pool brush to scrub the walls, floor, and corners. Algae can stick to surfaces, especially in areas with poor circulation. Brushing helps to loosen the algae, making it easier to filter out.

5. Add an algaecide

If the algae persists after shocking, consider adding an algaecide. Different types of algaecides target various algae strains:

• Copper-based algaecides: Effective against green algae.

• Polymer-based algaecides: Best for mustard algae or other types.

Apply the algaecide after shocking the pool and brushing it. Allow the pool to circulate for 24 hours before swimming.

6. Clean your skimmer and filter

After the algae has been removed, clean the skimmer and filter. Clogged filters won’t function efficiently, so ensure that all debris is removed. Regular cleaning will help maintain water quality and prevent future algae blooms.

Preventing Green Pool Water in the Future

The best way to keep your pool from turning green again is to establish a routine pool maintenance regimen. Here’s how to prevent green water from occurring:

• Maintain chlorine levels: Always ensure your pool has the proper amount of chlorine. Test the chlorine levels at least once a week and adjust as needed.

• Check water chemistry regularly: Regularly check the pH, alkalinity, and calcium hardness levels to ensure the water remains balanced.

• Brush and vacuum your pool: Brushing the walls and vacuuming the pool at least once a week removes debris and prevents algae growth.

• Use a pool cover: During periods of non-use, cover your pool with a mesh or solid cover. This will keep debris out and reduce sunlight exposure, which encourages algae growth.

Additional Tips for Pool Care

• Monitor the water level: Keep the water level at the midpoint of the skimmer to ensure the filter works effectively.

• Remove debris after storms: Strong winds or heavy rain can introduce debris into the pool. Remove leaves, twigs, and other organic matter to reduce the risk of algae growth.

• Winterize your pool: If you’re closing the pool for the winter, make sure it’s properly winterized to prevent algae and debris from accumulating during the off-season.

Other Considerations

• Heavy rain: After heavy rainfall, pool chemistry can shift. Always check your pool’s water levels after a storm and rebalance the chemicals to prevent algae growth.

• Water source contamination: Sometimes, high levels of metals in your water source can lead to staining or discoloration in the pool. Consider using a metal sequestrant to help prevent metals from causing problems in your pool.