Update:
I had to go out of town for work this week so I left instructions with my fiancé about the feeding regime for the achilles. Got the news that it had stopped eating on Tuesday or Wednesday (to quote: "I dunno, I didn't look that closely". Sigh.), but since I'm the only one who knows anything about fish in the house and I was in doing field work out of cell range near Brooks, there wasn't much I could do to trouble shoot.
I got home late Thursday night after the lights in the tank were off but was told the Achilles had been out swimming around all day (but not eating). Woke up this morning to find it dead as a door nail.
I did a little post-mortem on it with my field scope and it looks like either a bacteria or fungus attacked it's mouth, like to the point where it's lower lip was practically gone. The upper lip was covered in this white fluffy goo that came off when I rubbed it, and seemed to be in the process of also eroding away. Needless to say I'm pretty devastated, as much because I lost the fish as because I could have probably done something about it had I been here and seen the infection when it started. I've got a hospital tank down stairs ready to go
I did a thorough inspection of him on Monday morning before I left and everything seemed normal, so whatever this was it moved fast. I talked to the LFS that I bought him from and have searched thoroughly online and the general consensus is that fungus is really rare in aquariums, so it's more likely that his mouth got injured and due to either a weakened immune system, stress, or both, he developed a bacterial infection. The other tang was still harassing him at night when the lights were preparing to go out (chasing him away from ALL the sleeping sights), so the chances are good he was a contributing factor. For all I know, the other tang was responsible for the original mouth injury.
So, the other tang is going. He's a cool fish, with loads of personality, but a bristle tooth tang was never on the list of 'must haves' and this aquarium is all about the 'must haves'. And the thought that a 40 dollar fish could have assisted in the demise of a 350 dollar fish makes me ill. I'm going to get another tang eventually, one I really want, and it's not going to have to deal with one second of harassment from an incumbent.
It took most of the day with the trap, but I finally caught him a few minutes ago, so if anybody is looking for a feisty little blue eyed bristle tooth tang in the absolute prime of health (borderline obese), he's yours.
Also, I know it was mentioned that a sump isn't appropriate for a tang, and in general I would agree, but my sump isn't a normal sump. The largest chamber in it is used for water changes and is over 50 gallons, which is larger than a lot of people's display tanks. He won't be in there long.