Milkiness Causes

• When water in a swimming pool starts looking milky or cloudy, there are usually several general causes. A common cause of milky water in a pool is that its pH or total alkalinity (or both) levels are too high. Another cause of milky pool water is that too-low pool chlorine levels have allowed contaminants from bathers to build to excessive levels. A third cause of milky pool water is often an ineffective or blocked-up filtration unit.
  
  

Pool Alkalinity

• Swimming pool alkalinity has to do with both pH as well as total alkalinity of the water. Pool pH should be maintained from 7.4 to 7.6 for optimum effectiveness. Total alkalinity in a pool should be held from 80 to 150 parts per million (ppm) to prevent dissolved minerals in the water from solidifying. When pool pH or total alkalinity in a pool is too high, dissolved minerals solidify into fine particulate matter that can create milky-looking water.
  
  

Ineffective Chlorination

• Swimming pools that aren't chlorinated to recommended levels of 1 to 3 ppm (2 ppm is best) often are overwhelmed by contaminants from swimmers. Pools under heavy usage by numerous swimmers that aren't properly chlorinated can turn cloudy or milky. An effective solution to such milky or cloudy water is to backwash (reverse) the pool's filter. After backwashing a pool's filter to clear milky water, the pool will need to be superchlorinated (shocked) to at least 10 ppm.
  
  

Poor Filtration

• Many pool owners using filters containing sand as their filtration material neglect to change the sand periodically. In a sand-based pool filter, it's easy for the sand's individual particles to become coated with calcium if the pool's water is too hard. If a swimming pool's filter sand is blocked or otherwise made inefficient, milky water often develops. Replace a pool's filter sand if it is coated by calcium (it'll look white), or use a filter cleanser to sharpen the sand backup.
  
  

Stabilization Problems

• A special problem that can create milky water in outdoor swimming pools has to do with high levels of chlorine stabilizers in the water. The most common chlorine stabilizer for pools is cyanuric acid (CYA). CYA works well in protecting a pool's available chlorine levels from the harsh sunlight. However, high levels of CYA in a pool make chlorine ineffective, which can lead to milky water. Maintain CYA levels in a pool at 30 to 50 ppm for maximum chlorine effectiveness.