It will be low on ALK, CA and possibly other components. It'll also leave white particles in the bottom of your mixing bucket. When salt turns hard a checmical reaction happens and some of the minerals are no longer disolvable. I like others have use it after breaking it up very well, but I mixed it in small portions with good salt to lessen the effect. It took quite a while to use a box I had this way, but never saw a problem in the tank.