Cloudy swimming pool water is caused by low Free Chlorine, debris, ammonia, young algae, 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 a cloudy pool caused by low FC levels, you must shock your pool by adding liquid chlorine to raise your FC to the recommended level depending on your water's Cyanuric acid (chlorine stabilizer) level.
The recommended level of FC is 2 ppm for 30 ppm cya and 3 ppm for 40 ppm cya and should never go below that; otherwise, your water will turn cloudy, which means you have chloramine or combined chlorine.
If you have a cloudy saltwater swimming pool, first measure your FC level. The recommended FC levels are 3 ppm for 70 ppm cya and 4 ppm for 80 ppm cya. If the FC level is below the recommended range and the percentage setting in your Saltwater Chlorine Generator (SWCG) is high, or your pump's speed is already high, you can't raise the 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 dull. Before assuming any cause of cloudy pool water, take an accurate reading of FC and make sure it's 3 ppm.
To clear cloudy pool water caused by debris faster, use Pool Clarifier to remove all the particles in your water through the filter and backwash your filter to clean it.
You can also use Poolfloc to collect all the particles at the bottom of your pool and then use a vacuum cleaner to remove the particles; the choice is yours.
3. Ammonia
Cloudy swimming pool water caused by ammonia is the most difficult to clear because you use a lot of chlorine to fix your pool. If you have ammonia in your pool, the 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 find their way into 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 a cloudy pool caused by ammonia, you have to get rid of ammonia by adding liquid chlorine to raise your FC to 10 ppm or higher. After adding chlorine, take the FC reading after 15 minutes and raise it back to 10 ppm when it falls below 5 ppm.
Repeat the process until the FC level settles between 5 to 8 ppm to be sure your pool is free of ammonia; leave FC to come down to 3 ppm and balance all your chemicals. Click get rid of ammonia using the link above for more details on how to eliminate ammonia in your pool.
4. Young Algae
The early stages of algae are indicated by a cloudy pool that lacks chlorine to sanitize water and kill bacteria-causing algae.
To test if algae are 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 you should start the treatment for algae.
5. Poor Filtration System
A poor filtration system is one of the major causes of cloudy water. Ensure that your filter is working as required by backwashing your pool filter regularly, especially when the pressure gauge rises abnormally.
Also, make sure you replace your filtering agent when worn out, mainly sand and DE filters, which may allow particles to come back to your pool.
It would be best to change the sand in your filter every 5 years for a home pool; however, for a public pool, you might need to change the sand after 2 to 3 years.
For better results than filter sand, you can use filter glass or D.E. Media + Filter Aid to remove very fine particles for more transparent pool water.
6. Poor Chemical Balance
Apart from FC, wrong chemical readings, especially the pH, Total Alkalinity, and Calcium Hardness will make your water look cloudy. An incorrect pH level will make your water look dull. Chlorine will also lose its effectiveness when the pH reading is wrong, and as a result, your water will be cloudy.
High Calcium levels may also cause scaling, and as a result, your water will appear cloudy. If accumulated in large amounts, other chemicals like phosphate will make pool water cloudy.
To avoid cloudy water caused by wrong levels of other chemicals like pH, Calcium, TA, and Phosphate, take an accurate daily reading of all pool chemicals using one of these reliable test kits and balance them as required.
7. Mineral Deposits
Mineral deposits, including Phosphates, Nitrates, Silicates, and Sulfates, among others, are some causes of cloudy swimming pool water that is difficult to clear.
These minerals can find their way into the pool from some pool chemicals we use, water that runs into a pool when it rains, and body oils, lotions, or soaps.
You can measure mineral levels frequently and use mineral removers like Natural Chemistry PHOSfree Phosphate remover to lower mineral levels in your pool and clear cloudy water.