Sorry to hear about your woes.:sad2:
If I was to give it my best guess, based on your post, It definitely sounds like an ALK issue. IME Alk levels too low will definitely cause rtn issues with sps.
If your Alk really went down to 5.3, it's not surprising to me that you're having issues. That's pretty low.
I'm really confused about the part where you hooked up a Ca. Rx. and your Alk went down? Just as a reference when I first hooked up a Ca. Rx. on my tank years ago, my Alk shot up to 14.0 before I knew it.(I had the opposite problem)
What what were you using for Alk and Ca before this Ca. Rx?
Sounds like you don't have it adjusted correctly or maybe it's too small for your tank?
Once you get it set up and adjusted correctly. it should keep your Alk level up no problem. You definitely need to test your levels often when you're first getting it dialed in to your system.
Anyway, If you haven't done so already, get that Alk level back up to at least 8.0.
Unfortunately it might take a few days for your corals to recover and stop receding.
You might have to frag some corals well ahead of the rtn to save you from loosing the whole coral.
Is the Ca. Rx. the only thing you change recently?
Did you run a new batch of GFO recently, and pull your Alk down even faster along with this new Ca. Rx?
Hope you can get things back on track.