ok..i was way off pool is 5yrs old. its 16x32, 20,000 gallons. my ph is is always between 7.2 - 7.6 my chlorine level is always 1.0 - 1.5. i use aqua chem shock..sodium dichloro-triazinetriorie. there is no visible algae. my local pool supply store says i need to change my sand every 3 to 4 yrs..confused?