The following is an excerpt from an excellent article: Mycobacterial Infections of Fish
"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. Because the disease can masquerade as a number of other conditions, samples should be taken from any fish showing signs of chronic disease or reproductive problems and analyzed with bench-top acid-fast staining.
The typical lesion caused by mycobacteriosis is granulomatous inflammation. Granulomas (Fig. 1) signify the body’s effort to isolate an irritant or foreign body of some sort. Typically, granulomas are recognized by the appearance of a wall of tissue around the area where the bacteria are active. Granulomas are often seen during microscopic examination of infected tissues, but in advanced cases
they may be visible to the unaided eye (Fig. 2). Although considered typical of the disease, granulomas do not develop in all cases."
Fig 1 is a slide of granulomas shown under a microscope.
Fig 2 is a photo of a fish's spleen, taken during a dissection of an infected fish.