Fish TB ussually is only externally identifiable in the advanced stages of the disease, symptoms are ussually white nodules on the body, scale loss, skeletal deformities, abnormal bulges. Early indicators are the usual for most bacterial diseases, loss of appetite, wasting, fin clamping, scale loss, eye bulge, associated fin rot.
Diagnosis of a mycobacterium infection is ussually easiest by performing a disection and microscopy of internal organs (looking for white/gray nodules (tuberculous cysts), these ussually primarily appear on the liver, kidneys and intestines).
Fish TB is actually pretty common, but the disease is not especially virulent under most conditions, and fish that are otherwise healthy can often keep an infection at bay for many years without showing visible symptoms.
Best course of action is to euthanize the fish showing the most obvious symptoms and try to diagnose via microscopy. If tuberculous cysts are found on internal organs, all other fish showing obvious symptoms should also be euthanized. Regular vacuuming and water changes should be employed to help lower bacterial load, antibiotics such as tetracycline, doxycycline and erythromycin may be somewhat effective, but don't count on them.