Actually, the powerheads wont really make a difference on or off in this situation. As long as their currents are not at any time hitting the sand, then it wont be kicking up any more than what is already floating about. When it was just sand and water in my tank i kept the powerheads on 24/7 and thats actually what cleared the tank up for me. And yeah, you really should rinse it extremely well. For aragonite, its best to rinse the sand until the water turns from a thick milky color, to being cloudy. Then you will know its good to put in the tank.