actually not just the sand inside... you have carbon, resin and sands with different grade also... and need no change for 5-10 years if you properly maintain it... just do backwash everyday.. how i know? I'm water treatment specialist in my factory... and we have even much bigger sand filter to treat the underground water... already use for more than 5 years! treated water still in good quality! and i can get FRP tank at cheap price (RM700 without installation)!