I've read the whole thread now. Really sorry to hear of your struggle. I agree with your actions to this point and I like your parameters. So, I am kind grasping at straws here. Hopefully, the ICP test provides a clue. While I've killed my share of acros over the years, "puking out guts" is a brand new visual for me. Is anyone familiar with a more precise term for this behavior? Is this what "expelling zooxanthellae" looks like? Or is this something else?
As to "grasping at straws":
- Some kind of toxin introduced from the air or a corroding part. I've heard many strange airborne stories... oven cleaners, burned teflon pan, air fresheners, you name it. Seems that the carbon would have trapped it tho.
- Stray current. This is a stretch; I manage a tank that had it for quite some time without harm. Voltmeter is cheap for checking that.
- Fish list? Angels? File fish? somebody going rogue? My angels ignore acros; they prefer expensive zoas - Rastas in particular
- A pest. IME tho, they prefer the base and eat their way up the branches. AEFW, red bugs and apparently black bugs (which I have never seen but)
- Chemical warfare from aggressive corals. I had a chalice once that...
Whatever the cause, I do recommend clipping off the dead skeleton all the way back to / including live tissue. None of mine have ever grown tissue back over dead skeleton. In fact my acros almost all respond very well to being cut back. I'll see multiple "buds" within two/three weeks at each cut.
Let us know about the ICP. Good luck.
When I say puking guts out you are right, I would consider it expelling zooxanthellae. To asnwer a couple more things:
- There was a corroding magnet on my Apex optical level sensor (which i've read is a problem with these) so I got rid of that the magnet bracket from Apex and got one from Octo Aquatics that is made 100 times better. There are so many magnets in tanks nowadays, including holding my Gyres on, that I don't know if I'd be able to find any and every one but did find the apparent one

As far as airborne, nothing new since I started my tank but do have a plug in air freshener near that I am going to move, just in case.
- You hear a lot of different things on stray current. Some swear that it's a problem, some swear it isn't. I think I'm going to grab a grounding rod just in case. It won't hurt anything and they aren't that expensive.
- Fish list is: 2 Pajama Cardinals, yellow watchman goby, royal gramma, orchid dottyback, azure damsel, Melanurus wrasse, orangeback fairy wrasse, Pintail fairy wrasse, Linespot flasher wrasse, Magnificent Foxface, Powder brown tang, Gem tang, two spot bristletooth tang, blue tang, blue jaw trigger. Before anyone gets too worried, the Blue tang will be moving to my 300 gallon when it gets a little bigger

No known coral nippers in the bunch.
I did a close examination for pests both with my camera and magnifying glass and don't see anything....
- Chemical warfare - non of my corals are close enough for this. I did have a chalice cause some damage to my scoly before i noticed and moved them apart. Chalice are mean.