Recommended hours of filtration = 12 or more.
The most common and easiest to maintain is a sand filter. This works by running water through a sand bed that is several inches thick. A sand filter can filter particles of dirt as small as 40 microns in diameter. A micron is 1 millionth of a meter. If you have good eyesight you can just see a particle that is about 40 microns in size. A human hair is approximately 75 microns in diameter. Any dirt smaller than 40 microns passes through a sand filter and remains in the pool water.
A lot of 40 micron particles that you can’t see doesn’t seem like it could cause a cloudy water problem until you get billions of them together in your pool. Then the water looks cloudy and the dirt just runs right through your filter and back into the pool. For a quick and easy demonstration, put your pool light on at night and notice the gray cone illuminated by the light. That is unfiltered dirt.
Solution? Drain and refill the pool or hire a pool service with a portable Diatomaceous earth filter. This is usually a cart with a pump, motor and DE filter mounted. They just put the inlet hose in one side of the pool and the outlet in another and let it run until the water’s clear.
Sand is not a permanent medium. It becomes contaminated with suntan oil and oils from the human body as well as dirt. Sand should be changed every 4 to 6 years (if you don’t know how long it’s been since the last change or you moved into your house without knowing how long and you have cloudy water, change it NOW!). Be sure to buy your sand at a pool store or a store that carries sand meant for sand filters. Beach sand or sand from the local plant nursery does not have the squared edges required for filtration and won’t work.