How to Fix a Green Swimming Pool

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When you open up your swimming pool this spring, you may be greeted by green water. The likely cause is algae, small micro-organisms that grow quickly in warm water. When chlorine levels drop, even for a day, algae results and can turn your pool water green.

Fortunately, algae is easy to kill and keep under control. Here’s how to fix a green swimming pool.

Chlorine Shock

Chlorine is a very effective water sanitizer. If the swimming pool is maintained with 3 parts per million (3pp) of chlorine, it will keep algae from growing. However, when the pool hasn’t been used for some time and the chlorine level drops, algae will develop.

Boosting chlorine levels will destroys algae. Chlorine pool shock with a main active ingredient of calcium hypochlorite is the fastest way to banish algae from your pool.

Shade of Green Determines Procedure

In order to effectively kill algae, you must first determine how much algae is in you pool. You can tell by the colour of the water.

  • Light Green or Teal-Coloured water means a low amount of algae. Double shock your pool water by adding two pounds for every 10,000 gallons of water.
  • Green or Dark Green water means a medium amount of algae. Triple shock your pool water by adding three pounds for every 10,000 gallons of water.
  • Black Green water is the worst. You will need to quadruple shock your pool by adding four pounds of shock for every 10,000 gallons of water.
  • Cloudy-Blue or White water means the algae is dead. Run the pump and filter until the water is clear. The time it takes to clear depends on the size of your pump. You may also want to use a pool clarifier to help the process along.

Timing is Key

Wait until dusk or night time to shock your pool. This is because the chlorine in shock is not stabilized and will not be as effective during the day because the sun will burn it off much faster than regular stabilized chlorine.


Before adding shock to the water, always pre-dissolve it in water to ensure that it will be properly circulated throughout the water. It also reduces the risk of bleaching a liner, if there is one.

A pound of shock treats up to 10,000 gallons of pool water. Each bag of shock should be pre-dissolved in its own five gallon bucket of warm water. Fill the bucket with water first, and then add the shock. Keep the pool filter and pump running for at least 10 hours after you shock.

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