Cara's Pool School — clarifier

Although your filter can be effective at removing most dirt and small particles from the water, it cannot possibly remove all possible impurities. Like the screens used on windows, there is a limit as to how much it can and must allow to pass through without becoming a solid surface. Over time, these tiny particles will accumulate in number and the water's clarity can be adversely affected (cloudy to view, or "dust" particles floating in the water when the light is turned on are common indicators). To assist the filter in dirt and debris removal, routine use of a clarifier...

Any dirt or debris should be removed as soon as possible to avoid potential damage or staining to the finish, along with cleaning the filter (backwashing) and emptying the pump and skimmer baskets. It's a good idea to also shock the pool and, if there is a persistent clarity issue, using a clarifier could speed up the filtration process.