Instead of adding chlorine to the water of a saltwater pool, you add salt, and a chlorinator converts it to hypochlorous acid (HOCl). This chemical is sometimes referred to as free chlorine, and it's the actual sanitizing agent in both saltwater and conventional pools.
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When the water turns cloudy, it usually means there's an HOCl deficiency, and that could be the result of insufficient salt levels or problems with the chlorinator. The problem might also be pH, which is the first thing you should check. If the pH is outside the optimal range of 7.2 to 7.6, free chlorine generated from the salty water doesn't last long enough to do its job.
Balance the pH
The pH value is a measure of the acidity/alkalinity of the pool water; a reading of 7 means the water is neutral. Pool water should ideally be slightly alkaline, with a pH between 7.2 and 7.6. If it's higher than this, the alkaline water quickly neutralizes the hypochlorous acid generated by the chlorinator. In acidic water with a pH lower than 7, hypochlorous acid reacts too quickly with contaminants and gets used up faster than the chlorinator can produce it.
Before addressing chlorine deficiency, it's important to raise or lower the pH, as needed, to bring it within the proper range. Lower pH by adding muriatic acid or sodium disulfide to the water, and raise it by adding baking soda (sodium bicarbonate) or soda ash (sodium carbonate). Check the total alkalinity of the pool water before raising pH. If it's near the acceptable range of 80 to 120 ppm, use soda ash. Otherwise, use baking soda, which has a stronger effect on alkalinity.
Measure the Salt Level
The optimal salt level in the pool depends on the chlorinator, so you should read the manual to find out what it should be. Salt is corrosive, so you don't want to add too much, or the pool liner, circulation equipment and your skin will suffer. In most cases, the ideal level is 3,000 parts per million, which is about one-tenth as salty as seawater. When adding salt, stir it into the water, and then let the water circulate for an hour before taking another measurement.
Adjust the Chlorinator
If the pH and salt level are in the proper ranges, but the free chlorine level is below its ideal range of 1 to 3 ppm, you may need to increase the output of the chlorinator. Most models have a super-chlorination setting, which can slowly raise the chlorine level to 5 ppm and higher. This isn't the same as shocking the water, but it may clear the water. Beware, though: Repeatedly using this feature shortens the life of the chlorinator.
Clean the Chlorinator Plates -- Chlorinators consist of a pair of electrolytic plates, and these eventually get covered over with scale, especially if the water has a high calcium content. Scale reduces the electric charge between the plates and the output of the chlorinator. Clean the plates by removing them and washing them off with clean water. If the scale is heavy, you may need to immerse the plates overnight in vinegar to dissolve it.
Shock the Water
The water in saltwater pools -- like that in conventional ones -- needs to be shocked periodically to kill microorganisms that manage to survive in normal chlorination conditions. Shock the pool by adding enough sodium hypochlorite in the form of pool chlorine, bleach or a conventional shock product to quickly raise the free chlorine level above 10 ppm. Wait for the level to fall below 5 ppm before using the pool.
If It's Still Cloudy
Some cloudiness may persist in pool water after you shock it. This is usually due to dead microorganisms, mineral deposits and other inert contaminants. You may be able to clear these by introducing a water clarifier, which coagulates these contaminants into clusters large enough to be trapped in the pool filter. In severe cases, or when you don't have time to wait for a clarifier to work, use a flocculant. It creates larger clusters that fall to the bottom of the pool, which you can remove with the pool vacuum.