Cloudy water is typically the result of several factors each of these should be addressed to clear a cloudy pool. The following steps should be taken in this order.
Poor Filtration/Circulation: The pump and filter of a pool are designed to remove large particles from the water. If they are not functioning properly or long enough the water is not being cleaned. You cannot correct a cloudy pool until these are functioning. Make sure filter media is clean; backwash sand filters, clean and rinse off cartridge filters. For optimal circulation point directional eyeballs in return fittings downward on a 45 degree angle.
The filtration and circulation system may not be able to keep up with the dirt in the pool. Make sure that the skimmer and pump baskets are clean and free of debris and the pool has been vacuumed.
Water Balance will affect water clarity. High pH, high total alkalinity, high calcium hardness, and high cyanuric acid will cause cloudy water in several ways. Have the pool water tested and balanced into the correct ideal ranges. Balancing the water can sometimes clear a cloudy pool.
A low sanitizer level will permit the formation of Algae. A cloudy pool can represent the beginning of an algae bloom. If the Sanitizer is low or zero increase the chlorine level by shocking or oxidizing the pool with triple your normal dosage.
Clean filter system. Backwash the sand filter or thoroughly rinse cartridge filter to remove captured algae. A chemical filter cleaning treatment is recommended after heavy algae infestations.