So unfortunately, this is heading toward a sad ending. After 2 days in the acclimation box with all the fish ignoring him, I released the anthias last night and everything seemed fine. This morning though, he'd fully claimed the main cave and became really aggressive toward my blue assessor and especially Sushi, my tilefish (that lots of you already know is the star of my tank).
I tried putting up a mirror, which he attacked for a few hours, but eventually he went right back to harassing other fish. I was also feeding heavy every two hours, so there was no way he was hungry, but that didn't help at all. Sushi had several full on panic episodes and hit the lid a few times.
Tilefish truly can't handle any aggression, even if it's not targeted at them, as he'd freak out if the anthias was chasing the blue assessor too.
Anyway, after watching things get worse and getting worried about Sushi hurting himself, I managed to lure the anthias back into the acclimation box (with the mirror, go figure). Sushi has been mostly staying in his cave, but overall seems much calmer now.
At this point I don't think reintroducing the anthias will work, and I'm pretty sure I should've gotten a smaller less boisterous specimen. I just read everywhere these anthias were peaceful fish, so I didn't think the size would matter.
Here's a video I put together so you can see how bad it got earlier today:
