I usually start running the carbon if my skimmer get nuts
If you don't have reactor put some carbon in mesh bag and put into filter sock to get good flow thrue it
Dead snail/crab/fish/other clean up crew members? I've had big die offs of coral a couple of times due to something that doesn't register on our "normal" reef tests. Once was dead animals, and the other was when I tried to start an algae scrubber and somehow got brass and aluminum in my sump. Check for something like that and definitely start with some carbon now
Okay, so last night I threw some carbon in my sump in a bag as suggested. This morning polyp extension is starting to look a little better. So far no acro loss/rtn. Thank goodness! I hope this disaster has been avoided. Don't know what the heck happened, but I am convinced it was something with the new refugium I started running (and have since disconnected). I will not be changing anything on this tank again! Things were happy before and I just want to get back to that
I'll probably run the carbon for a few more days. I'm not going to do any further water changes at this point because I want to try to keep the tank parameters as stable as possible to hopefully keep the acros happier. I like the suggestion to start running carbon any time the skimmer starts going nuts. That makes sense. I don't want to run carbon continuously.
I'd retest phosphates.
What testkit did you use? Try to check with another.
Having Added Ceramic frag racks as you say holds a chance of phosphate release as well as the dying macro.
Macro alga could have released some toxins as well that ticked of your SPS.
Would definitely Run Carbon, continue with water changes and if possible add a heavy metal removing resin or sponge in case you can't pinpoint anything and corals still look bad...