Mycobacteria are slow-growing, environmentally persistent organisms that produce granulomas in the internal organs of infected fish. The disease is chronic: a fish may carry it for months, gradually losing condition, before anything is obvious, and by then other fish in the system are usually infected. Diagnosis requires histopathology or culture, and culture is slow. There is no treatment regime that reliably clears it in an aquarium, and the organisms are resistant to many common antibacterials. The human health aspect is the part hobby sources under-report: infection through broken skin produces a persistent nodular lesion, usually on the hand or forearm, which is frequently misdiagnosed because doctors do not routinely ask about aquaria.
Short answer
Mycobacteriosis is a chronic bacterial disease producing slow wasting, spinal curvature and sometimes ulcers, over months rather than days. It cannot be diagnosed without laboratory work, there is no practical cure in an aquarium setting, and — uniquely among the conditions here — it can infect the person keeping the fish through broken skin.
Mycobacterium marinum causes 'fish tank granuloma' in people: a persistent nodular skin lesion, usually on a hand or forearm, entering through a cut or abrasion during tank maintenance. It is slow to develop, slow to resolve, frequently misdiagnosed because it is uncommon, and requires specific and prolonged antibiotic treatment. Wear waterproof gloves if you have any break in the skin on your hands, do not put damaged skin into aquarium water, wash thoroughly afterwards, and if you develop a persistent skin lesion tell your doctor you keep an aquarium — that single sentence is often what makes the diagnosis. People who are immunosuppressed should take particular care.
Needs veterinary diagnosis
Establishing this requires laboratory work — culture, histopathology or necropsy. Anything short of that is a suspicion, and should be described as one.
Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.
Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.
How to tell it apart: A fish that is out-competed at feeding time wastes in the same way. Watch feeding closely and try target-feeding for two weeks. A fish that regains condition was hungry, not infected — and this is much the commoner explanation for a thin fish.
How to tell it apart: Consider how long you have had the fish and what its species' lifespan is. A four-year-old neon tetra losing condition is probably reaching the end of its life. This differential is routinely overlooked in favour of a disease explanation.
How to tell it apart: A developmental curvature is present from early life and does not progress. Mycobacterial curvature appears in an adult fish and worsens.
How to tell it apart: Parasitic wasting may show visible worms at the vent, and typically responds to appropriate worming. Mycobacteriosis does not respond to worming.
How to tell it apart: Ordinary bacterial infection is usually acute rather than developing over months, and frequently responds to appropriate antibacterial treatment. A chronic wasting picture that does not respond raises the suspicion of mycobacteriosis, which is why non-response is diagnostically meaningful.
Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.
AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.
There is no treatment regime that reliably clears mycobacteriosis in an aquarium. The organisms are resistant to many common antibacterials, treatment would be prolonged, and attempting it risks selecting resistant organisms with a documented human health dimension. AquariumHQ does not suggest one.
Good water quality, appropriate feeding and reduced stress can extend a fish's comfortable life, and are appropriate where the fish is still feeding and behaving normally.
Where a fish is wasting badly, no longer feeding, or has developed spinal deformity, humane euthanasia is the appropriate outcome and should be discussed with a veterinarian. Prolonging a wasting illness is not kindness.
For confirmation of the diagnosis, for advice on the system as a whole, and for humane euthanasia. Confirmation matters because it changes how you handle the tank and because it has implications for your own health.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.
Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.
Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.
Slow-growing mycobacteria including Mycobacterium marinum, producing chronic granulomatous disease in fish
Last verified 2026-08-11
Mycobacterium marinum causes cutaneous granulomatous infection in humans, acquired through broken skin during aquarium contact, and is frequently misdiagnosed without an aquarium history
AquariumHQ regards the under-reporting of this risk in hobby sources as a genuine safety failure, and states it prominently rather than in a footnote.
Last verified 2026-08-11
Diagnosis requires histopathology or culture; clinical signs alone are not sufficient to distinguish it from other causes of chronic wasting
Last verified 2026-08-11
No treatment regime reliably clears mycobacteriosis in an aquarium setting; management focuses on biosecurity and welfare
Last verified 2026-08-11