Every three to four months you should change out the water because standing water attracts bugs and bacteria.  Take your plant out of the pot.  Wash the rocks off in very hot water.  Rinse the pot out in hot water.  Examine the roots to make sure they are a healthy white, yellow or orange color.  Then put it all back together again and add clean new bottled or purified water.  In some areas you can use the tap water if you allow it to sit opened for two days.