are you sure your calcium level is low? high light also can prevent coralline algae from growing, as will alkalinity swings. alkalinity swings can be caused by adding something like purple up.
if your calcium level is low, i would suggest using a 2 part additive. check out bulk reef supply. they are about the cheapest for additives.
there is also a product called b-ionic.
i would invest in some quality test kits first however. salifert would be my choice.