I just started working taking care of a commercial pool and spa. I want to know how often or when to clean my DE filter grids. They are in a tank on the suction side of the pump. I mounted pressure and vacuum gauges for the system. When filters are clean the pressure reads 18 PSI and the vacuum is at 8". The previous person said to clean once or twice a week but in pool school they said that frequent cleaning, when not needed, is a waste of time and money for the DE powder. Tell me what you guys/gals think.