I dose ESV Alk., Cal., and Mag. I do not add any supplements to my tanks that I can not test for. I do not use Purple Up or any similar product for that reason. IMO, those products are a waste of money. They may buffer your calcium, but there has to be proper balance between Alk., Cal., and Mag. to keep your parameters stable. If one of those three chemicals fall out of the acceptable range it can have adverse effects on the other two, therefore making your water parameters unstable.
What are the levels of your Cal. Alk. and Mag. right now? Your tank may not have a high demand for calcium and alkalinity due to the types of corals that you keep. Frequent water changes may be all it takes to keep your parameters in the acceptable ranges.