Its around 25,700 gallons and I'm not sure the size of the pump and I'm not home to check. I'm still working on gathering information for this site. We changed the sand 2 years ago.What size and how old is the filter? What size pump? Sand seldom needs changing, but may be low in the tank as there is always some (very little, but some) lost with each backwash especially if the pump is oversized.