Sand is by far the easiest to maintain but its filtering ability is the lowest - down to about 30 microns. This can be particularly evident if your pool is lighted at night. Filter aids such as DE powder or cellulose fiber, however, can help improve sand's filtering ability. The filtering is done by the sharp edges of the sand granules which trap the dirt between them. As the water flows over the sand these edges get 'polished' and rounded off and the filtering ability is lessened over time. New sand filters better than old sand! Ask any aquarist who has ever used a sand filter! (I'm talking mechanical filtering here, not biological which is not applicable to pools.) Pools are no different. In fact some of the filters are identical! For example, Pentair makes sand filters for large aquariums and ponds as well as pools. Funny thing is they are exactly the same! Cleaning a sand filter is easy - when the pressure reaches a certain level (see owners manual) simply backwash the filter to remove the trapped dirt and debris. Back-washing is about a 10 minute process. The constant back-washing will tend to throw your water balance off but this can be useful if you use trichlor or cal hypo. Sand does occasionally need a more thorough cleaning than back-washing alone can do. Perhaps once a year, I recommend opening up the filter, sticking a garden hose in the sand and turning it on. Let the water flush out all the crud that has collected in the sand until the overflow is clean. You might have to CAREFULLY break up the sand with something like a broom handle if it is clumped together! (I said CAREFULLY so you don't damage the laterals!) If there is evidence of scale buildup then an acidic sand cleaner might be in order!
) Sounds like this filter may need a sand change