Holy crap!
In all honestly once the corals adapt 1500 shouldn't be too high, but I would try and keep it under 1400. Also too much magnesium at once could introduce a bit of ammonia. I have never measured any, but if you raised your tank by 200 ppm all at once that could end up with big problems. Of course if the damage is already done then do water changes.
Many recommend only raising magnesium by a max of 50 ppm per day. I would suggest 50 ppm per week, to be on the safe side.
Did you try my method of only dosing your newly made up salt water?
Once you get that method figured out it works really really well. Best of all you never dose the tank only newly made up water.
Look at post #2.