Water changes are not the way to go....unless your salt mix has higher than normal alk (most don't). Because the organisms in your tank are using up the alk, as you've noticed, it immediately starts to drop, so unless you do 100% water changes every day, it will never stay at normal levels.
I use Ca, Alk & Magnesium additives from bulkreefsupply. They are very cheap compared to things like B-Ionic.
-R