What's wrong with my tang?

[sdm]

Okay, a while ago (read: about a week ago) my kole tang began to act oddly, that is, it began swimming using only its pectoral fins and sinking when stationary. Now, I know it sounds stupid in hindsight, but as school was filling up nearly my every waking moment, I, for some reason I can not understand, put off looking on here for help, writing it off as "probably temporary" and saying I probably couldn't do anything to help by myself anyway. Well, looking into the tank last night, I clearly saw the "hope it's just temporary" plan wasn't going to work out as my tang has suddenly become practically emaciated. Does anyone have any ideas as to what could be going on?

 

[JHemdal]

My guess is that the fish has just lost energy and become emaciated over time due to malnutrition - this is common enough in tangs, especially small ones.
A fish that becomes emaciated overnight (and not just that you missed the gradual onset over a few weeks) can really only be a problem with osmotic balance. Usually, with kidney failure, the fish develops edema, and fills up with fluid (ascites) and becomes bloated. In the case of severe osmotic stress (such as being kept at high salinity or sudden rise in salinity) a fish will dehydrate and literally become bone thin overnight. Can't tell you why this might have happend....

Jay Hemdal