From Mycobacterial Infections of Fish:I'm beginning to think that my tanks have some form of mycobacterium. I have had. Few mysterious death/illnesses. I'll explain one by one....
Guppy ------ Definitely some sort of tuberculosis.... This fish had extreme bloating, and lesions.. It was like this for around two weeks, i then euthanized as it was the most humane thing to do. This escalated slowly.... MAY 2012
Guppy------- I'm not sure on this one. He got a bit bloated and then all of a sudden died, within two to three days. OCTOBER 2012
Betta -------- He started to bloat really bad, and was on the bottom of the tank for a while. Sakura helped treat with metronidazole, worked like a charm. Came back on Monday night, died Tuesday. OCTOBER 2012 AND JANUARY 2012
Platy ------- On Tuesday night, had raised scales, fin rot, and white bit on her, all of a sudden. Died Wednesday morning. JANUARY 2013
Platy and two guppies were from same store and lived in same tank. I use all the same supplies.
"Mycobacterium causes a chronic disease, usually characterized by wasting. It should be suspected when fish are in poor condition and also have scale loss, skin ulcers, or a history of reproductive problems. Occasionally, deep hemorrhagic skin lesions will be seen in addition to the more common superficial lesions....."
The article says that "the disease can masquerade as a number of other conditions," and the only way to determine whether it's a Mycobacterial infection is by having the fish analyzed by a lab. (They open it up to examine the internal organs, etc.)