The introduction of foreign matter coupled with pool filter operation may cause cloudy water (algae may also cause cloudy water). Check the filter run pressure, if it is over 10psi higher than the start-up pressure after cleaning then the filter is not operating effectively and needs to be assessed for either cleaning or element replacement.
A high pH level can also be attributed to the cause of cloudy water. Your pH should be checked and adjusted weekly as part of your general pool maintenance.
Another factor which can cause cloudy water is poor circulation or low filtration times. Some filters/pumps require to be be run continuously, such as above ground pools. Some pool owners are reluctant to do this and mistakenly blame the lack or excess of chlorine for the cloudiness.
Poor circulation occurs most frequently when the main drain is turned of and the pool is run purely on skimmer filtration. The skimmer circulates only the surface water which results in a build-up of contaminants in the deep water of the pool. Main drains should always be in operation except when using an automatic pool cleaner through its own suction outlet, if using a pool cleaner through the skimmer, the main drain must be open for suction.
Bob Charlton is a Certified Pool & Spa Operator and Inspector with the National Swimming Pool Foundation with 15 years of experience of pools in Florida.