Alikat,
If you could post a picture, that would help. If not, about the only likely determination would be that the fish had HLLE, Head and Lateral Line Erosion that afflicts this species (as well as many others).
You are likely to discover that there are many, many proposed causes for this, and just as many "cures". Much of this is inaccurate, misleading or just plain wrong.
After 40 years of dealing with this malady, I have three pieces of information I bleieve is true:
1) The vast majority of full "cures" involved the moving of the affected fish to a new aquarium, one that has not had a prior issue with HLLE.
2) Of all the anecdotal cures, IMO the only one with any merit is boosting the fish's diet quality by enhancing it with the best foods available. If nothing else, it can't hurt. I will not go so far as to say that a poor diet causes HLLE, just that improving a fish's diet can reduce its affects. Having worked with multiple tanks at a public aquarium, I've seen fish fed the same diets, yet fish in certain tanks develop HLLE and others do not...which in my mind rules out diet as a cause.
3) The use of carbon has been proven to induce HLLE in fishes. It isn't, as once supposed, because the carbon removes essential elements from the water, but rather, something about carbon dust that may irritate the fish's skin. Carbon used in tanks with powerful skimmers (which remove carbon dust) rarely see this problem. Dusty, unwashed carbon used in a tank with poor mechanical filtration is the worst offender. I won't go so far as to say most HLLE is caused by carbon dust, but much of it is. In our collection of 2000+ fishes, since we stopped using carbon six years ago, the number of fish we have with HLLE dropped from dozens to one single fish, with a minor case, that we've had for about 10 years.
Jay
