From what I understand, if your alk is low, you shouldn't be dosing calcium, you should be dosing alk. If your magnesium is at 1500, that is a little high, but not terrible. I definitely wouldn't be trying to boost it. With that much magnesium, you should be able to get your alk back up. What is your calcium level at?
Also, what is in your system for corals and inverts(such as clams)?
What is the magnesium level and calcium level of the new salt water?
What is the PH of your tank water and the new salt water you mixed? Alk can have a bearing on the PH. Here is a link to alkinity and PH.
http://www.advancedaquarist.com/2002/5/chemistry