Well right now all my corals are upset including my old ones. Only gsp which doesn't die and zoa's which are new doing well. I'm going to go slow. Carbon. Water change. ICP and act. Nothing too dramatic too fast.Every once in a while when coral warfare from soft corals (I suspect this is the issue) happens and all the other corals look angry, it sounds like what you may have going on. Or, you say the older corals are doing ok and the newer additions are the ones noticeably struggling. You may have an element(s) built up over time that the older corals have been able to slowly acclimate themselves to whereas the newer have not had that same slow build up.
Either way the ICP and carbon are probably your best bet for now. I get notably better results from carbon when I use it in a reactor like a Phosban 150 or similar.
If it were me, I would put the carbon in it he area where I will get the best pass thru of the bag, also I keep media like PolyFilter and TLF Metazorb (or Cuprisorb) for cases where metals may be the culprit.
Good luck, and don’t change to much to fast unless they all look to be suffering.