Aquarium hygiene: mycobacteriosis, and what a tank can pass to people

Quick answer

The equipment that touches tank water should touch nothing else, and never the kitchen sink.

Aquarium water carries bacteria that live in biofilm, and one group of them, the atypical mycobacteria, causes a chronic wasting disease in fish and a slowly growing skin infection in people. Both are avoidable, and both are missed for months because neither looks urgent early.

Nothing here should stop anyone keeping fish. The routine that removes almost all the risk is short: never start a siphon with your mouth, cover any break in the skin, keep aquarium equipment separate from household equipment, and wash your hands and forearms afterwards.

• Fish tank granuloma is a real human infection, and doctors rarely think of it unless you say you keep fish.
• Mycobacteriosis in fish has no reliable cure, so the decisions are about containment, not treatment.
• Fish that waste away one at a time over months in an established tank are the classic pattern.
• Anyone immunosuppressed in the household should not be doing tank maintenance.
• Cleaning aquarium equipment in a kitchen sink moves the problem to where food is prepared.

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Never start a siphon by sucking on the tube.

It puts aquarium water directly into your mouth, along with everything living in it, and it is the single riskiest habit in the hobby. Use a squeeze primer bulb, a shake-to-start siphon, or a small pump. They cost very little and they remove the exposure completely.

Do not put hands or forearms into a tank if the skin is broken. A cut, a hangnail, a scratch or a patch of eczema is the route these organisms use, and they cannot get in through intact skin.
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Decide in 60 seconds

Where the organisms actually live

Not in the water column, which is what owners tend to picture. They live in biofilm, the slippery layer that coats every surface in a mature aquarium: the substrate, the filter sponges and media, the inside of hoses, decor, plant leaves and the silicone in the corners.

That is why a water change does not remove them and why a clean-looking tank is not a reassurance. It is also why the organic load matters: the more waste accumulating in the substrate and filter, the more biofilm there is for these bacteria to live in.

Two things follow. Routine maintenance genuinely reduces the load, because gravel vacuuming and filter servicing physically remove organic material. And porous items that have been in an affected system are very difficult to disinfect properly, so replacement is usually safer than cleaning.

Rinse filter media in used tank water rather than under the tap. Chlorine and chloramine kill the nitrifying bacteria you rely on, and losing those causes a far more immediate problem than the biofilm you were trying to reduce.

Mycobacteriosis in the fish

Colour loss and increasing time spent resting on the bottom are early and easily dismissed.

The disease is slow, which is exactly why it is missed. It progresses over months and takes fish one at a time, so each loss gets its own separate explanation.

Early.
Gradual weight loss in a fish that is still eating. Loss of colour. More time spent resting on the substrate or hanging in a corner. Reduced interest in swimming against flow. Nothing here is specific, and every one of these signs is more often something else.

Established.
Weight loss becomes obvious from above, with a ridge along the back and a hollow behind the head. Skin ulcers appear that do not respond to the usual antibacterial treatments. Fins erode. One or both eyes may protrude. The abdomen can swell as internal organs are affected.

Late.
Spinal curvature, either an arched back or a sideways bend, in a fish that has not been injured. This is the sign most keepers eventually recognise, and by then the fish has been infected for a long time.

At post-mortem.
Pale nodules through the liver, spleen and kidney. This is where the diagnosis is actually made, from tissue examined by a laboratory, which is why confirming it means submitting a fish that has died or been humanely euthanised.

What it is not.
Internal parasites also cause wasting, but the wasting fish usually passes pale, stringy faeces and responds to treatment. Simple competition at feeding time produces a thin fish that recovers when it is fed separately. Old age produces decline in a fish that has reached the top of its expected lifespan. Chronic poor water quality affects the whole tank rather than one fish at a time. A bent spine can also come from a mineral or vitamin shortfall, from a genetic deformity present from birth, or from an electrical fault in the water. What separates mycobacteriosis is the combination: months, one fish at a time, no response to treatment, and wasting alongside ulcers or spinal change.

Treatment.
There is no reliable cure. Antibiotic courses may suppress signs temporarily, rarely clear the organism, and encourage resistance in bacteria that also infect people. Treating the whole system does not work, because the reservoir is the biofilm.

What it looks like in a person

The human form is usually called fish tank granuloma. It follows the same rule as the fish disease: slow, and dismissed early.

It begins at a break in the skin, almost always on a hand, finger or forearm, and it takes weeks to appear. By the time the lump shows, most people have forgotten the scratch that let it in.

The lesion is typically a firm reddish or purplish nodule that grows slowly and often does not hurt much. It can ulcerate. In some cases a chain of further nodules develops in a line up the arm, following the lymphatic drainage.

It stays superficial in a healthy person because these bacteria prefer temperatures below human core temperature. That is why the hands and forearms are affected and why it does not usually become a systemic illness in someone with a working immune system.

The practical problem is diagnostic delay. A slow-growing lump on a hand is not an obvious infection, and the laboratory only finds the organism if it is asked to culture at a lower temperature and for a longer period than routine. That request is made when someone mentions the aquarium.

Treatment is a course of specific antibiotics, usually long, sometimes several months, and occasionally with surgery. Getting there quickly depends almost entirely on one sentence at the appointment.

Who is at higher risk

Anyone taking immunosuppressive treatment, including chemotherapy, transplant medication and biologic drugs for inflammatory conditions.

Anyone with HIV, poorly controlled diabetes, or chronic liver disease. Liver disease matters specifically for marine and brackish systems, where a different group of organisms poses a much more serious risk.

Pregnant people, very young children, and older adults.

Anyone with eczema, dermatitis or chronically broken skin on the hands, because the barrier that normally protects them is not intact.

In these households the answer is not to give up the aquarium. It is that the higher-risk person does not do the maintenance, and that gloves and hand hygiene are treated as compulsory for whoever does.

The other organisms in tank water

Aeromonas.
Common in fresh water and a recognised cause of wound infection after a scratch or cut sustained during tank work.

Vibrio species.
Relevant to marine and brackish systems. Some cause severe infection through broken skin, particularly in people with liver disease, and these can progress far faster than mycobacteria.

Erysipelothrix.
Causes an infection of the hands known to people who handle fish commercially, presenting as a spreading purplish area of skin.

Salmonella.
More often associated with reptiles and amphibians, but present in ponds and in systems shared with turtles or visited by wild birds.

None of these change the routine. The same measures prevent all of them, which is the useful thing about this list.

The routine that removes almost all the risk

Dedicated equipment.
Buckets, siphons, nets, sponges and jugs used for the aquarium and nothing else. Label them. A bucket that is also used for household cleaning defeats the purpose in both directions, since detergent residue is lethal to fish.

Never mouth-siphon.
Use a primer bulb, a shake siphon or a pump.

Cover broken skin, or use gloves.
Full-length aquarium gloves that reach above the elbow are inexpensive and are the single most effective item here. Waterproof dressings work for a small cut.

Wash hands and forearms afterwards.
Soap and running water, including under the nails, every time, before you touch food, your face or a phone.

Do not clean aquarium equipment in a kitchen sink.
Use a bathroom, a utility sink or outside. Dispose of tank water down a toilet or an outside drain rather than into a sink used for food preparation.

Keep frozen and live fish foods separate.
Sealed, in their own container, ideally in a separate box within the freezer, and wash your hands after handling them.

One set of tools per tank, or disinfect between tanks.
Sharing a net between an established tank and a quarantine tank is the standard route by which a problem in one system becomes a problem in all of them.

Quarantine new fish and new plants.
This is the measure that stops the organism arriving in the first place, and it matters more than anything you can do afterwards.

Keep the organic load down.
Vacuum the substrate as part of water changes, service the filter on a schedule, and avoid overfeeding. Less accumulated waste means less biofilm.

If a tank is affected

Stop adding fish. A new arrival into an affected system is a new casualty.

Do not sell, swap, donate or rehome fish, plants, snails, water, substrate or equipment from that system. This is how the problem spreads between keepers, and it is usually done in complete good faith.

Get a diagnosis rather than guessing, because the management is different and much more restrictive than for a treatable disease. That means submitting a fish for laboratory examination through a vet or a fish health service.

Decide about the system with a clear head. Some keepers continue with a closed tank, adding nothing, allowing the existing fish to live out their lives with good water quality and no new introductions. Others break it down. If you break it down, replace porous items rather than trying to clean them, because these organisms are relatively resistant to disinfection.

Euthanise affected fish humanely rather than letting a wasting disease run its course. A fish with spinal deformity and open ulcers is suffering, and there is no treatment coming.

What never to do

Do not start a siphon with your mouth, under any circumstances.

Do not put unprotected hands into a tank with a cut, a hangnail, a scratch or broken skin from eczema.

Do not clean aquarium equipment in the kitchen sink or tip tank water into it.

Do not treat a wasting fish with repeated courses of antibacterial products bought over the counter. It rarely helps the fish and it selects for resistance in organisms that also infect people.

Do not move fish, plants or equipment from a tank with unexplained chronic losses into another tank or another home.

Do not share nets, siphons or algae scrapers between an established tank and a quarantine or hospital tank.

Do not ignore a lump on your hand or arm because it does not hurt. Painless and slow is exactly how this presents.

Do not stop keeping fish because of this article. The exposure route is narrow and the precautions are simple.

When to get help

See a doctor for any lump, nodule or non-healing sore that appears on the hand, finger or forearm of someone who keeps fish, however small and however painless, and tell them about the aquarium at the start of the appointment rather than at the end.

Go promptly if a lesion is spreading, if a line of lumps is developing up the arm, or if the person affected is immunosuppressed, diabetic or has liver disease. In marine and brackish keepers, any rapidly worsening wound infection after tank contact needs urgent medical attention rather than an appointment next week.

For the fish, seek a veterinary or fish health service when a tank is losing fish one at a time over months, when fish waste away while still eating, or when skin ulcers do not respond to treatment. Ask specifically about submitting a fish for laboratory examination, because that is the only way the question gets answered.

Bring the dated record of additions and losses, your water test results, the age of the system, and what has already been treated. That history is what turns a guess into a diagnosis.