It is common in danio i'm afraid. There not much to be done. If he seems happy just let him be for now.
Symptoms:
Because of the symptoms associated with this disease, it is often referred to as Wasting Disease. An infected fish may show a loss of appetite, emaciation (sunken belly), fading of colors, eroding fins, erratic swimming, scale loss or protrusion, "pop-eye" or eye loss, skin inflammation, ulcerous skin wounds or open lesions, gill deformities, spinal curvature, and Dropsy.
Symptoms may occur singly or in various combinations. Symptoms may also vary from species to species and from one individual fish to another. Not all symptoms need be present. Healthy fish may carry the illness for some time without being affected, and then become ill when stress or poor water conditions lower their resistance. The disease may run a lingering course, killing the fish slowly over time, or strike in epidemic proportions quickly wiping out an entire aquarium population.
Diagnoses of Piscine Tuberculosis is difficult, as all of the diseases symptoms may appear in other illnesses. Piscine TB can only be verified upon autopsy.
Cause:
Mycobacterium bacteria. Piscine Tuberculosis is highly infectious and can be easily transferred. Gravel can harbor this bacteria causing the entire aquarium to become infected. The disease may strike in epidemic proporations, killing an entire population of fish in record time with little to no symptoms. However, it can also remain latent for some time, progressing slowly, silently causing internal organ damage to the fish.
This illness is not always fatal to the fish. The bacteria may become encapsulated to form small nodules and as long as good environmental conditions are maintained there is no danger. However, if the fish is weakened by unsuitable water conditions or other diseases the nodules can burst. The infection then becomes acute and can kill the infected fish as well as infect others.
Treatment:
Infected fish must be isolated quickly because the disease is highly contagious (see below). Treat with a combination of sulphafurazone (0.2mg/g fish), doxycycline (0.005 mg/g fish) and minocycline (0.005 mg/g fish) administered intramuscularly. It is also recommended to feed any sick fish isoniazid. It may take up to 2 months for fish to completely heal. Kanacyn also claims to be helpful in treating Piscine Tuberculosis.
Caution:
Piscine Tuberculosis is caused by a species of bacteria belonging to the genus Mycobacterium, which is also the causative agent of Tuberculosis in humans. While the bacteria that causes this disease in fish prefers cooler temperatures than most bacteria that infect humans it is still possible for the illness to be passed on to humans. Such an infection in humans usually shows in the form of an infected nodule in the skin, although there is a chance of a more serious internal infection. If you suspect your fish has this disease, observe the strictest of hygiene to prevent the spread of this serious disease to humans.
