Thanks Gwen,
The fish guy just left with another big ugly crab. We suspect this one was the culprit since we found his lair in the proximity of the tang's "bedroom" rock. Also, we found a disturbingly high number of empty hermit crab shells right where it lived. I wonder if that thing has been eating my hermits. I'm glad it's out.
The little tang is doing better. He's quite a trooper! We substituted the rock for a piece of
pvc pipe (the rock was a bit big for the basket). He is quite disoriented but his eye socket does not show signs of infection. Hopefully he will start eating soon. He has been renamed "Mad Eye Moody"; it was the fish guy's idea. I did not know he was a Harry Potter fan

But I like the new name, and I share the fish guy's cautious optimism for his recovery.
I'll keep you posted.
Ann