Cloudy swimming pool water is caused by low Free Chlorine, debris, ammonia, poor filtration system, and wrong chemical levels.
1. Low Free Chlorine(FC)
Low FC is the most common cause of cloudy/milky/murky/dull pool water. To clear cloudy pool caused by low FC levels, you need to shock your pool by adding liquid chlorine to raise your FC to recommended level in relation to Cyanuric acid(chlorine stabilizer) level in your water.
The recommend level of FC is 3ppm and should never go below 3ppm otherwise your water will turn cloudy, which means you have chloramine or combined chlorine in your water.
If you have a saltwater swimming pool and it's cloudy; firstly, measure your FC level. If FC level is below 3 ppm and the percentage settings in your Saltwater Chlorine Generator (SWCG) is high or your pump's speed is already high and you can't raise FC level in your pool, you need to shut down your SWCG and shock your swimming by adding chlorine manually like a non-saltwater pool.
2. Foreign Particles or Debris
Foreign particles, debris, or dirt may cause your pool water to turn cloudy or look dull. Always before assuming any cause of cloudy pool water, take accurate reading of FC and make sure it's 3ppm.
To clear cloudy pool water caused by debris faster, use Pool Clarifier to remove all the particles in your water through the filter and then backwash your filter to clean it.
You can also use poolfloc to collect all the particles at the bottom of your pool then vacuum up the pool to remove the particles, the choice is yours.
Cloudy swimming pool water caused by ammonia is the most difficult to clear because you use a lot chlorine to fix your pool. If you have ammonia in your pool, FC level will not rise after adding a lot of Chlorine and FC settles at 0ppm or near zero in a short duration, your Cyanuric acid level will be 0ppm or near zero, and your Combined Chlorine(CC) will be very high.
Ammonia is caused by bacteria that finds it way in your pool due to low FC levels. Ammonia feeds on Cyanuric acid and that is why your Cyanuric acid level will be 0ppm or near zero.
To clear cloudy pool caused by ammonia, you have to get rid of ammonia by adding liquid chlorine to raise your FC to 10ppm, take FC reading after 15 minutes and raise it back to 10ppm if it falls below 5ppm, repeat the process until FC level settles betwee 5 to 8ppm to be sure your pool is free of ammonia, leave FC to come down to 3ppm and balance your chemicals. Click get rid of of ammonia in the link above for more details on how to eliminate ammonia in your pool.
4. Young Algae
Early stages of algae is indicated by cloudy pool that of course lacks chlorine to sanitize water and kill bacteria causing algae.
To test if you have algae starting up, shock your swimming pool in the evening and take free chlorine reading the following morning: If your FC level reduces by more than 1ppm the following morning, you have algae and should start treatment for algae.
5. Poor Filtration System
Poor filtration system is one of the major cause of cloudy water. Ensure that your filter is working as required by backwashing your pool filter regularly when pressure gauge rises abnormally.
Also make sure you replace your filtering agent when worn out especially sand and DE filter that may allow particles to come back to your pool.
You should change the sand in your filter after every 5 years for a home pool, however, for a public pool you might need to change the sand after 2 to 3 years.
6. Poor Chemical Balance
Apart from FC, wrong chemical readings especially the pH, Total Alkalinity, and Calcium Hardness will make your water look cloudy. Wrong pH will make your water look dull. Chlorine will also lose its effectiveness in wrong pH reading and as a result your water will be cloudy.
High Calcium levels may also cause scaling and as result your water will appear cloudy. Other chemicals like Phosphate if accumulated in large amount will turn pool water cloudy.
To avoid cloudy water caused by wrong levels of other chemicals like pH, Calcium, TA, and Phosphate, take accurate reading of all pool chemicals on daily basis using one of these reliable test kits and balance them as required.