Most minerals in tap water are not available to the orchids.Orchids are slow growing,slow to use nutrients.Calcium must be applied though means of calcium nitrate or dolomite.Dolomite usually take 4months or more to actually be useful to orchids.If a foliar calcium supplement is used you will see results in weeks not months.Calcium is one of the slowest moving nutrient in plant tissue.A foliar supplement is the best way to get calcium to your orchids.