Calcium Questions

I have a calcium question. I am going on 4 weeks now with a calcium reading in my tank of 500. The mag is 1290 and Alk 9.9. All have remained here for the month. pH ranges from 7.95-8.15 depending on time of day. I use seachem reef salt, do 10 gallon weekly changes in the 120 gal display, 40 gal sump (using 20 gal of available capacity). I have added nothing in the form of additives or anything other than food. I did add about 10 pounds of rubble marco rock, uncured, dry to the sump about the same time I noticed the spike to 500 in the calcium. I have done nothing and will continue to do nothing to correct the high calcium. I have a few real sweet frags that will start arriving next week and just want to keep things stable above everything else and not have things in limbo when they get here.
Anyone else experience this issue of high calcium? Is it possible the salt mix is off and causing it? I am nearing the end of a bucket and have heard of inconsistancy in mixes before. Any words of advice from those of you full of wisdom? Thanks.