You don't need to change your sand unless you had baquacil. Most of us have had our sand for many years/Your water is probably cloudy due to algae. You need to get your chlorine up to shock, which at a cya of 40 would be 15ppms. You need to check your chlorine level 3x a day and get it back up to 15 each time to kill off the aglae - do this until your chlorine holds overnight. It may take a couple of days.