The water from any good filtration system (RO or other) will have alot of the Cal/Mag removed as that's a primary function of the system. If your nute say mix with distilled or filtered water, you should have no worries as the manufacturer will generally put in enough to make up the difference. If you are supposed to use tap water, you may wind up a bit short and want to add a drop. Of course all this depends on the plants you are growing. Some strains are cal/mag hogs while others couldn't care less.