Solubility isn't an issue with the water (to any practical extent). It is an issue with the salts. Gypsum is not that soluble in water but if you are trying to dissolve more than wants to you are using too much. Calcium carbonate is essentially insoluble in water unless you supply carbonic (or another) acid and that's why you shouldn't try to dissolve calcium carbonate in RO (or other) water unless you are supplying carbonic acid which is definitely for advanced brewers only (who wouldn't, for the most part, choose to go that route).