Mysterious Attack on Firefish?

[nosferatu51]

I was walking by the tank right before bed and saw my purple firefish out for the 1st time in since I put him in the main tank (2-3 days ago). When I looked closer he looked completely stressed out and then I noticed his head. It looked as though somebody/thing had skinned it. His body is fine, but his head looks kinda messed up. I put him in a hanging breeder thing I have with a large shell for shelter, but I highly doubt he'll make it til morning. He had that look like people do in horror movies where they stumble around after having their arms ripped off.
I don't get it....I'm assuming aggression from somewhere because it doesn't look disease like. I can't imagine anything in there would bother him though. The purple tang and the bluejaw trigger should both be very easy for him to get away from (not to mention he always hid when they swam by anyway). The bluejaw has been asleep for hours and when they were in the QT I never saw the purple tang make any sort of aggressive move towards the firefish or the trigger. The only other critters that I know of in there are hermits, snails, and a skunk cleaner shrimp. I haven't heard any mantis clicks...although I think I saw an emerald crab the other day (though I've had 4 of them and none have ever been aggressive....but I've never had them with fish that hide like a firefish does). Emeralds wouldn't do that though would they? The emerald crab must have come on the rock I got from someone locally and I can't imagine a mantis having been in their tank for 2 years and them never having noticed.
I had the other member of that firefish pair die in the QT tank mysteriously with no physical indicators. Good one night then the next day I went to put them in the main tank and a corpse fell out of the rock. The LR that the crab would have come from wasn't in that tank though which makes me think it had to be something else.
How has he gone from perfect health to this in a few days? Anybody got any ideas?

 

[nosferatu51]

Just to add, he and the other 2 fish were in QT for 2 weeks before they got added to the tank. The 1st firefish died towards the end of it, but everything seemed fine so I added the 3 of them to the main tank. I figured maybe something weird had happened to the one and it was just a freak accident since there was no observable pathology. I'm at a loss here on this one and I don't know whether I should assume disease or aggression and whether I should be worried for the other 2 fish in the tank.

The tank is new, but it was definitely cycled. I checked the levels just this morning actually. 0 nitrites and ammonia, pH was a little low 7.9ish (I hate my pH test, it's hard to read the color changes), and nitrates were about 10ppm if not less. SG is 1.025 by refractometer.

I have a modded Maxijet in there....could he have hurt himself on that? God I wish I knew what happened at least....losing that much money on a pair of fish in addition to losing what were probably my favorite fish in the tank (the pair of purple firefish were AWESOME) is really bothering me.