There are four basic reasons why pool water can look milky or cloudy. The methods of clarifying the water, including using chemicals, will vary, depending on the cause of the cloudiness. You can also cause your pool to look cloudy simply by adding chemicals that are incompatible.
The term "pH" is a measurement of whether water is acidic or alkaline. When the pH of your water is greater than 7.8, meaning it is highly alkaline, the chlorine acts more slowly and is less efficient at killing the algae that can cause cloudy water. Your water should have a pH between 7.4 and 7.6, which is comfortable to the eyes of swimmers.
The term "precipitation" used when discussing swimming pools is when a chemical reaction in the water creates small particles. If the water in your pool has high pH, particles of floating dissolved salts can make it look milky or cloudy. Add bicarbonate of soda to raise alkalinity. Add sodium bisulfate or muriatic acid to lower alkalinity. Add your acid slowly in a solution no stronger than eight parts acid to one part water.
If the filter of your pool is not working properly, the water may become polluted by dirt, sweat and lotions from the bodies of swimmers. You need to clean your filters and then superchlorinate your water. The North Carolina Department of Environment and Natural Resources recommends sodium hypo, lithium hypo, dichlor or calcium hypo to superchlorinate water until it contains 10 parts per million (PPM) of free chlorine.
Sunlight dissipates hypochlorous acid, causing the loss of the free chlorine needed to keep your water clear. You may be adding cynauric acid, called a stabilizer, to slow the loss of chlorine. Keep levels of cyanuric acid below 60 PPM; more than 100 PPM can cause “chlorine shock,” turning your water hazy. To correct chlorine shock, drain part of your pool, and then superchlorinate to 10 PPM of free chlorine.
If your water contains more than 400 PPM of calcium, it has calcium hardness. If the water is warmer than 85 degrees Fahrenheit and you have calcium hardness, your water may look cloudy. If your water has a high level of calcium, use a settling agent to eliminate some of the calcium and reduce the amount of calcium-based chlorine that you might be using. If your calcium level is very high, drain your pool and fill it with fresh water.
You can cloud your water by adding chemicals that are incompatible. You will cloud the water if you simultaneously add a water clarifier and a chemical to inhibit the buildup of lime scale. You will cloud it if you simultaneously add chlorine and scale inhibitor. You will also cloud your water if you simultaneously add soda ash and a chemical to increase the hardness of the water.