Fish Tuberculosis
Symptoms
Chronic, slow progression over weeks to months. Wasting (hollow belly, spine prominently visible despite eating). Spinal curvature, fin erosion that doesn't respond to standard antibiotics. Eye protrusion or cloudiness. Skin ulcers that won't heal. Sudden death of multiple older fish in an established tank with no obvious water-quality cause is a classic Fish TB pattern.
Causes
Mycobacterium marinum (or related species) — slow-growing acid-fast bacterium. Lives in biofilms, substrate, and dead organic material in every aquarium. Becomes pathogenic in immunocompromised fish — chronic stress, poor diet, old age. Highly contagious between fish via ingestion of infected tissue or water transfer. Can persist in a tank for years.
Treatment
There is no reliable cure. Mycobacterium is intrinsically resistant to most aquarium antibiotics and the treatment regimens used in human TB take months, are expensive, and rarely succeed in fish. Best practice: cull affected fish humanely, perform deep gravel cleaning, and accept that the tank is a carrier. Removing all fish and breaking the tank down (substrate replaced, hardscape bleached and dried for weeks) is the only way to truly eliminate it. Many experienced keepers simply manage their best — keep husbandry pristine to minimize new cases.
Prevention
Quarantine all new fish for 4-6 weeks minimum. Don't share water, decor, or nets between tanks. Avoid feeding live foods from natural waters. Cull obvious carriers early before they expose others.
Notes
Fish TB is one of the few aquarium diseases that can infect humans — typically via cuts on hands or arms exposed to tank water. Causes a chronic skin lesion called 'fish tank granuloma' that requires months of human antibiotics. Wear gloves if you have open cuts and have any reason to suspect Fish TB in your tank.