Jason,
How big is your tank? Did you add the Mg all at once (like dump it into the tank or sump), or did you add it bit by bit in a high flow area in the tank, or what?
I don't have a sump to add my additives to, so I calculate how much additive I need with the
The Reef Chemistry Calculator, and add it 10 mls at a time with a baby medicine syringe into high flow areas in my 120g. Have never had any coral burning from adding buffer, Ca or Mg with this method.