identify this to please

In addition to my other identification post, my friends has also lost a yellow tang, a hog fish and the mentioned unidentified fish in my other post (possibly decorated rabbitfish). The tang was seen fine and happy, two hours later all trace of it has gone. Last addition of live rock was a year ago. They have taken out every piece of live rock and gone over it and tried to look in all the holes but theyfound nothing that could be doing this. Any suggestions?

 

"lost" fish usually have died for one reason or another, and have been dealt with by the cleanup crew before the disappearance is noted. By dealt with, I mean dealt with, not "some was left over". Flat out gone.

 

She told me she has high nitrates but not dangerously so. And they had had the tang for two years the hogfish and other for less and they all 3 disappeared within 2 weeks.

 

need more info on tank and specs