Mycobacteriosis (fish tuberculosis)

UrgentBacterial infection

Mycobacterium spp.

Also called: Fish TB, False fin rot, Wasting disease

Careful — humans can catch this

This infection can pass to people through cuts on your hands. Do not work in the tank with open wounds, and if slow-healing nodules appear on your hands, tell your doctor you keep an aquarium.

What you see

A slow disease unfolding over weeks and months: the fish wastes away despite eating, the back hollows, the belly may swell on an already thin fish, non-healing ulcers appear, paleness, sometimes a bent spine. Fins can crumble too — older literature calls it “false fin rot.”

How to tell it from its look-alikes

The separator from dropsy is time and body condition: dropsy is acute bloating on a normal-bodied fish, while mycobacteriosis is slow wasting that swelling, ulcers or deformities join over weeks. It differs from ordinary fin rot in that the fins erode WHILE the whole fish declines, and the water can be spotless.

Dropsy

The pinecone test from above is the main separator: a swollen belly with FLAT scales is usually overfeeding or a female's eggs, while raised scales mean fluid building up inside — organ failure. If the fish swelled slowly, over weeks, while wasting away — look at mycobacteriosis. A one-sided, slowly growing bulge suggests a tumour instead. Worth knowing: dropsy is a sign, not a disease — many causes produce it, most often a bacterial infection in failing kidneys.

Fin rot

The three common causes leave different tracks. Tankmate nipping — clean, scissor-like notches with no eroding rim, and it continues only while the nipper is present (the planner warns which species nip fins). Bacterial rot — an uneven, crumbling edge with a rim, usually in poor water. And fins going together with slow wasting of the fish points to mycobacteriosis, which likes to impersonate fin rot. Which cause is yours is settled most reliably not by how the fins look but by a water test.

The dangerous mistake

Months of medicine courses. No effective treatment for fish mycobacteriosis exists — not on the shop shelf and not in the veterinary formulary — and every extra week of treating is not only suffering for the fish but your own exposure time, because this infection passes to humans through skin abrasions.

Water first

In a young tank, most “disease” is not disease — it is ammonia or nitrite poisoning, and no medicine treats that. Before buying anything, test ammonia, nitrite and nitrate. If you keep the cycle log, the answer is already there.

New tank syndrome

What to do

Isolate the sick fish and, as it declines, euthanise humanely — the only path veterinary sources support; the clove-oil protocol is described in the dropsy entry, and a live fish never goes into the freezer or the toilet. Work in the tank only with gloves or fully intact skin — no open cuts in the water and never start a siphon by mouth. People with weakened immune systems should avoid contact with such a tank entirely. After an outbreak, disinfect equipment with 70% ethanol — ordinary disinfectants and bleach do not reliably kill these bacteria.

What to expect

Incurable in fish. For you: if slow-healing nodules appear on your hands weeks or months later, see a doctor and say you keep an aquarium — the incubation period runs from 3 weeks to 9 months, so doctors do not make the connection on their own.

This reference helps you recognise and tell conditions apart — it does not make the diagnosis. Only a microscope or a veterinarian confirms one. Use medicines exactly per the label, ideally in a quarantine tank, because several of them kill filter bacteria.