If using IO salt stop and use premium reef salt that is my advice! Also using Kent Essential elements will help raise your calcium levels because magnesium is required. Do not do what your LFS said that is insane solution.
Please read the following article:
An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System by Randy Holmes-Farley - Reefkeeping.com
Dosing Kalkwasser will help bring your Calcium and KH into line, for help with kalkwasser please read this.
What is Kalkwasser? and how do I apply it to my reef aquarium?