I have been at 500 before when I dial my reactor up for the growing corals. ...sometimes I go has high as 550, then it lowers to 425, or so as the corals grow.
There is something else that ****ed your corals off. Did you mix it up long enough? Was the temp OK? How do you dose?
How much water did you change? If you were at 425 and you are at 550 now, you would have had to changed 50% water with calcium at 675 to get up that high.