Why Is My Pool Green? 7 Common Causes and How to Fix It

It's finally getting warm outside and you can't wait to jump in for that first swim of the season. But when you go to uncover your pool, you're met with a big disappointment - instead of beautiful blue water, your pool is a murky green color!

Don't panic if you find your pool has turned green. While it may look unappealing and be unsafe to swim in, green pool water is usually caused by an imbalance in your pool's chemistry that allows algae to grow. With some troubleshooting and TLC, you can get your pool looking crystal clear again.

1. Algae Overgrowth

The #1 cause of green swimming pools is an overgrowth of algae. When the weather heats up, algae can quickly bloom and take over, turning your water into a slimy green soup.

There are a few different types of algae that can grow in your pool:

  • Green algae forms a green stain on pool surfaces. This is the most common type.
  • Black algae looks like dirt and often grows in spots.
  • Mustard algae creates yellow-green clouds in water.
  • Pink algae forms pink or reddish coats on surfaces.

No matter the type, algae loves warm, sunny weather. The hotter it is, the faster algae can reproduce and spread. Areas of your pool that receive the most sun will usually show algae first.

If your pool turns green overnight or after just a few hot days, algae is likely the culprit. You'll also find green stains or patches on the pool floor and walls. Running your pool pump and filter for 24 hours straight may help clear some algal blooms through filtration. But to fully treat green water from algae, you'll need to:

  • Shock chlorinate the water. This quickly kills algae.
  • Scrub walls, floors, and tile lines to remove algal growth. Be thorough!
  • Run your filter continuously for 24 hours afterward to remove dead algae.
  • Brush often and maintain proper chlorine levels to prevent regrowth.

2. Low Chlorine Levels

After algae, insufficient chlorine is the second most common reason for green swimming pools.

Chlorine is a powerful sanitizer that kills algae and bacteria that contaminate water. When chlorine levels drop too low, contaminants can quickly grow, causing green, cloudy water.

Here are some reasons your pool's chlorine levels may dip too low:

  • Hot weather causes chlorine to burn off faster.
  • Heavy bather load depletes chlorine reserves.
  • Failure to test and add chlorine regularly.
  • Improper chlorine level maintenance.
  • Chlorine gets used up fighting large algae bloom.

Test kits make it easy to check the free chlorine level in your pool. This should be kept between 1-3 ppm typically. If your test reads less than 1 ppm, simply adding chlorine can often clear up green water in a day or so if algae hasn't taken over yet.

Shock chlorination, which briefly raises chlorine to 5-10 ppm, can quickly kill algae and bacteria in green water caused by low levels. But you must also scrub away growth and get back to a normal maintenance routine.

3. High pH

For chlorine sanitizers to work their best at killing contaminants, your pool's pH must be kept in the proper range. If pH is allowed to creep too high, chlorine becomes very ineffective.

The ideal pH range for pools is between 7.2 and 7.8. Above 7.8, chlorine starts to become extremely unstable. Even if the chlorine level tests fine, high pH renders it basically useless as a sanitizer.

As chlorine loses efficacy, algae and bacteria can grow unchecked, turning water green. Additionally, high pH can cause other issues like:

  • Scale formation on pool surfaces and equipment.
  • Cloudy water.
  • Eye and skin irritation for swimmers.
  • Damage to metals in the pool and equipment.

Test strips or kits make checking pool pH fast and easy. To lower high pH, muriatic acid or dry acid are added carefully to the water. Getting your pH down between 7.2-7.8 will restore chlorine's sanitizing power.

4. Excess Pollen

Tree and plant pollen can be another contributor to green swimming pools in certain times of the year.

When wind and rain carry pollen into your pool, these tiny particles turn the water a greenish hue. Pollen tends to affect outdoor pools more since it falls directly into the water. But indoor pools can get pollen too through ventilation systems or on swimmers' bodies and suits.

Pollen algae is most common during spring, when trees and other plants are blooming. But grass pollen in summer and fall can also discolor water.

You'll notice pollen pool water is still fairly clear, unlike the cloudy, murky look of algae blooms. To treat green water from pollen:

  • Run your pool's filter continuously to remove pollen particles.
  • Use a clarifying agent or flocculant to clump pollen so the filter grabs it.
  • Clean out filter regularly to remove trapped pollen.
  • Maintain proper chlorine levels to prevent algae growth.
  • Use a pool cover when not in use to block additional pollen.

With heavy pollen, it may take a few days for the filter to fully clear the water. But pollen alone won't cause major water quality issues if you stay on top of filtration and chlorine levels.

5. Poor Circulation

For clear water, your pool pump needs to circulate all the water through the filter regularly. When circulation is poor, contaminants can build up and algae takes over, turning the water green.

Here are some common causes of inadequate circulation:

  • Old, worn out pool pump that can't move water well.
  • Clogged or dirty filter restricting water flow.
  • Skimmer baskets full of leaves and debris.
  • Pool pump running too infrequently or for too short a time.
  • Suction leaks lowering pump effectiveness.

Low water flow due to poor circulation also leads to dead spots in the pool where water sits stagnant. Algae loves to grow in these areas because sanitizers can't penetrate.

To improve circulation and prevent green water:

  • Clean all baskets and filters.
  • Inspect pump and filters for damage.
  • Ensure pump is running per manufacturer recommendations.
  • Check for leaks causing suction loss.
  • Consider upgrades if equipment is old.

Boosting circulation and water turnover will help stop algae growth in its tracks and carry contaminants to the filter for removal.

6. Filter Issues

Your pool's filter system is crucial for capturing contaminants and preventing algae growth. When filters can't do their job properly, green water is inevitable.

Some common filter problems include:

  • Clogged cartridges or sand filters.
  • Torn filter elements.
  • Missing or loose filter components.
  • Old media that needs replacing.

Filters that aren't cleaned or maintained regularly become less effective at trapping algae, bacteria, and organic matter. These accumulate in the water, causing cloudiness and green tint.

To fix green water caused by poor filtration:

  • Take apart and thoroughly clean filter per manufacturer instructions.
  • Inspect for damage and replace if needed.
  • Ensure proper reassembly.
  • Run filter for extended times to clear water.
  • Stick to schedule for cleaning and maintenance.

7. High Phosphates

Phosphates are another contaminant that can contribute to algae growth and green water if allowed to build up.

Sources of phosphates include:

  • Fertilizers and grass clippings entering the pool.
  • Leaves and debris that accumulate.
  • Swimmers' sweat, urine, and body oils.
  • Some pool chemicals like stain removers.

Phosphates provide nutrients that algae feed on. The more phosphates in the water, the faster algae can grow after being introduced.

A phosphate test kit can measure levels in your pool. If phosphates are above 100 ppb, use a phosphate remover per package instructions to lower the level. This will deprive algae of the nutrients it needs to bloom.

Be diligent about preventing phosphates from getting into the water and routinely test and treat for them. This will help deter algae growth and keep your pool sparkling.

Don't Let Green Pool Water Ruin Your Summer

Seeing that first hint of green in your pool at the start of swim season is disheartening. But with a little troubleshooting and TLC, you can get your water clean and ready for fun within a few days.

Be sure to test for improper chemical levels, clean filters, scrub away algae, and run equipment for extended times. Avoid swimming until water is clear to prevent health issues.

Staying on top of your pool's regular maintenance and chemical balance is key to preventing a recurrence. With the proper routine in place, you'll be able to enjoy a gorgeous green-free pool all season long.