Fish TB (tuberculosis in fish): signs, human risk and what to do

Fish TB, or fish tuberculosis, is a slow bacterial disease that makes fish waste away, and there is no reliable cure. It can also infect people through cuts on the hands, so wear waterproof gloves whenever you have broken skin and put your hands in the tank.

The disease is caused by Mycobacterium bacteria, most famously Mycobacterium marinum. Vets usually call it "mycobacteriosis" because it is not the same as human tuberculosis. It cannot be confirmed by eye, and many of its signs look like other problems. This guide helps you decide what is likely, how to protect the rest of your fish, and when to see a doctor yourself.

How to identify it

Fish TB is chronic, which means it develops over weeks or months. Common signs include:

Wasting. A thin fish with a sunken belly, often while it still eats.
A bent spine. A curve sideways or up and down that develops over time.
Sores and ulcers on the skin, loss of scales, and frayed fins that do not heal.
Faded colour or patchy pigment.
Popeye (bulging eyes) or a swollen belly in some fish. See popeye and dropsy.
Lethargy and loss of appetite near the end.
A slow trickle of deaths, one fish every week or two, rather than a sudden die-off.

Inside the fish, a vet or lab sees small grey or white nodules (called granulomas) in the kidney, spleen and liver. Some infected fish show no outward signs at all.

Many other problems cause the same signs, including internal worms, poor diet, old age and other bacterial infections. Only a lab test can confirm fish TB, usually on a dead or euthanised fish.

When it's a problem

Normal: An old fish slowly losing condition near the end of its natural lifespan.
Early warning: One fish is thin with a slight curve in the spine, and the rest look fine.
Serious: Several fish waste away over a few months, sores do not heal, or spines bend in more than one fish.
Human health concern: You develop a red or purple bump on a finger, hand, elbow or knee that does not clear up, especially after handling the tank with a cut. See a doctor (details below).

What to do right now

Protect your hands. Wear waterproof gloves (long ones are best) for any tank work until you know more. Cover cuts. Wash hands and arms after any contact with tank water.
Remove the sick fish to a hospital tank. Remove dead fish immediately, so others cannot eat them.
Test the water for ammonia, nitrite, nitrate and pH. Poor water and low oxygen make outbreaks worse.
Stop sharing equipment. Give the affected tank its own net, bucket and siphon.
Don't siphon by mouth. Use a siphon with a bulb or pump.
Keep a freshly dead fish for testing if you want a diagnosis. Wrap it in a damp paper towel in a bag and refrigerate it (do not freeze). Contact a vet or lab the same day.

What to do next

Treatment for the fish

Researchers and fish vets agree that there is no reliable cure. Mycobacteria grow slowly, hide inside the nodules, and need long courses of antibiotics. Some antibiotics have reduced signs in lab fish, but they have not reliably cleared the infection, and some strains are resistant. Antibiotics also need a vet prescription in many countries. Do not pour general antibiotics into the main tank. It will not cure fish TB and can harm your filter bacteria, shrimp, snails and plants.

What helps most is good care: clean, stable water, regular water changes, a good diet, and fewer fish per tank. Healthy fish are better at keeping the bacteria in check.

A fish that can no longer eat, swim or breathe comfortably should be humanely euthanised. See our dropsy guide for a humane method.

If a lab confirms fish TB

Treat the whole tank as exposed, because fish that look healthy may still carry it. You have three main options:

Keep the tank closed. Add no new fish, move no fish out, and let the current group live out their lives with excellent care. Many hobbyists choose this for a tank of older fish.
Break down the tank. Humanely euthanise the fish, throw away porous items, and disinfect everything else before you start again. This is the choice fish farms and vets usually recommend, especially if you breed or sell fish.
Get advice. An aquatic vet can help you decide based on your fish and household.

Never sell, trade or give away fish, plants, shrimp or snails from a confirmed tank.

Disinfecting equipment and tanks

Mycobacteria are tough. They live in biofilm (the slime on surfaces) and resist many common disinfectants. In one lab study, 70% ethyl alcohol and certain iodine and phenol disinfectants killed M. marinum within a minute, while household bleach needed about 20 minutes of contact to kill it. Some popular aquarium and farm disinfectants did not work well in that test.

Scrub everything first. Disinfectants do not reach bacteria under slime and dirt.
Soak nets, siphons and tools in a suitable disinfectant for the full contact time, then rinse well and air-dry.
If you use bleach, keep surfaces wet with it for at least 20 minutes, rinse thoroughly, and treat with dechlorinator before reuse. See chlorine.
Throw away porous items that are hard to clean: filter sponges, substrate, wood and plants.
70% alcohol is flammable. Use it away from heat and electrics.

Cautions for other animals

Shrimp, snails, plants, plecos and other catfish, and loaches are all harmed by many disinfectants and medicines. Never disinfect a tank with animals still in it.

Products that help

Long waterproof gloves: protect cuts on your hands and arms. Use them every time you work in an affected tank.
Liquid water test kit: helps you keep water quality high, which reduces stress and outbreaks.
Separate nets and buckets for each tank: stop you carrying bacteria between tanks.
Bulb or pump siphon: avoids mouth contact with tank water.
70% alcohol or a lab-grade iodine or phenol disinfectant: for soaking tools and wiping surfaces after scrubbing.
Hospital tank: lets you isolate sick fish and watch them closely.

How to prevent it

Quarantine every new fish for 4–6 weeks. See our quarantine guide.
Remove dead fish quickly so others cannot eat them.
Avoid overcrowding and keep oxygen levels high.
Be careful with live foods such as tubifex worms, which have been linked to outbreaks.
Use separate equipment for each tank.
Wear gloves when you have cuts, and wash hands and arms after tank work.

Why it happened

Mycobacteria are common in water, soil and biofilm, including many aquariums. Fish usually catch the disease by eating infected material, such as a dead tankmate or contaminated live food. Stress, crowding, low oxygen and poor water let the bacteria take hold. Imported or new fish can bring it in without showing signs.

When to get help

For your fish

Contact an aquatic vet or fish diagnostic lab if several fish waste away, if you breed or sell fish, or if you need to decide whether to break down a tank. A lab test is the only way to be sure.

For yourself

Fish TB can cause a skin infection in people called fish tank granuloma (also called swimming pool granuloma). It usually starts 2–3 weeks after exposure, sometimes longer, as a red or purple bump that slowly grows. It is most common on fingers, hands, elbows and knees. It can turn into a sore, and sometimes new bumps appear in a line up the arm.

See a doctor if you get a skin bump or sore that does not heal, especially if you have had a cut while working in a fish tank. Tell them you keep fish, because this infection is often mistaken for other conditions and needs specific tests and a long course of treatment. People with a weakened immune system (for example from medication, cancer treatment, diabetes or HIV) are at higher risk of serious infection and should talk to their doctor about safe fishkeeping. This guide is not medical advice.

In the game

Multiple Tank Syndrome doesn't have a fish TB card yet. The "Quarantine tank" card is the closest defence.