Algae

• Algae is a common reason why pool water gets cloudy. Algae can become a problem in a pool if proper sanitation and oxidation levels are not maintained. Usually a normal dose of chlorine or other sanitizer is enough to keep the various types of algae at bay, but extreme heat, excessive use or various chemical imbalances in the water can cause algae to grow and begin to cloud the water. If the problem persists, the water can begin to turn green or yellow as a result. Shocking the pool with chlorine to kill the algae or using a specially formulated algaecide will take care of the problem and will likely get your pool back to normal.
  
  

Poor Filtration

• Poor filtration can lead to cloudy water as impurities and various particles circulate through the filter system and clog it. A dirty pool filter will result in improper filtration and will leave too much of the impurities in the water. Even if the pump is pulling these particles through a sand filter, overcapacity could force it to release some of the particles back into the water, causing the clouding. Luckily, cleaning out the filter on your pool is a simple process. Your pump should have a setting called "backwash" on it. By selecting this setting, the pump reverses and pushes water through the filter backwards, cleaning out all of the particles in the filter and ejecting them outside the pool through a spill valve. Backwashing your filter a few times per season should keep this from occurring.
  
  

Lack of Use

• If you don't use your pool very much, lack of use could cause occasional clouding, which may be true even if you meticulously clean your pool and keep water chemistry balanced. If no swimmers get in the pool to stir up the water and keep it well circulated, clouding may result. Often this lack of use is the sole culprit. A pool that is used frequently and kept clean generally looks clearer and cleaner than one filled with semi-stagnant water that no one swims in.
  
  

Swimmer Waste

• One of the main causes of cloudy pool water is swimmer waste. Don't worry. The problem's not as bad as it sounds. People use all types of cosmetic products on their skin. Lotions, perfumes, deodorant and other products get washed off the body and end up in the pool's water. If your pool gets heavy use from many different people, you can reasonably expect they will contaminate the pool with some of these products. Shocking the pool regularly will help burn off these waste products and clear up the pool easily.