I agree with others that your Ca probably had nothing to do with it. I am curious about your ALK levels at the time of your low Ca levels. Those tend to work inverse to each other.
Your problem IMO, is either the ALK was to high, or that those two corals were very stressed to begin with and the dip in Coral Rx pushed them over the edge. @WWC recommends waiting 10 to 24 hours to dip corals after receiving them just for that reason. I use Coral Rx and have over done it with that dip as well. Lesson learned. I now error on the side of under dosing the Coral Rx in my dip, only leaving the corals in it for 2-3 minutes, and inspecting them carefully prior to putting them in my tank. I feel that if I need to dip again in a couple of days, I always have that option.
As far as the age of the tank, it may be a little young even though you are a stickler for parameters. I think there is more going on regarding bacteria and how SPS react than can be measured by the standard test kits we use.
Your problem IMO, is either the ALK was to high, or that those two corals were very stressed to begin with and the dip in Coral Rx pushed them over the edge. @WWC recommends waiting 10 to 24 hours to dip corals after receiving them just for that reason. I use Coral Rx and have over done it with that dip as well. Lesson learned. I now error on the side of under dosing the Coral Rx in my dip, only leaving the corals in it for 2-3 minutes, and inspecting them carefully prior to putting them in my tank. I feel that if I need to dip again in a couple of days, I always have that option.
As far as the age of the tank, it may be a little young even though you are a stickler for parameters. I think there is more going on regarding bacteria and how SPS react than can be measured by the standard test kits we use.