What is your daily Alk and Ca++ consumption ? How is your pH and what is your pH. And what is your PO4 and NO3 ?
Your real problem sounds like that tank is out of ionic balance and needs some large muti- WC, like 30 % 3 days in a row. I think the other issue is you "think" you are taking good care of the tank and are not. To much tinkering is not good, although one means well. I use to be like that. In other words, your tank is not stable
For the future if needed
A Guide to Using Calcium Reactors
http://reefkeeping.com/issues/2002-0...ture/index.htm
Calcium Reactor setup calculator (CaCO3/CO2 reactors)
http://home.comcast.net/~jdieck1/reactor.html