Mycobacterium fortuitum, as well as Mycobacterium marinum and Mycobacterium chelonae, are the etiological agents of fish Mycobacterioses. Mycobacteriosis has been reported to affect a wide range of freshwater and marine fish species, suggesting an ubiquitous distribution, and can cause zoonotic infections (known as "fish tank granuloma" or "swimming pool granuloma") in humans exposed to fish and contaminated water. Infection in human consists of nodular cutaneous lesions that can progress to tenosynovitis, arthritis, and osteomyelitis, depending on the immunological status. Authors describe some cases observed during routinary diagnostic activity in aquarium fish. Fish were sampled and histopathological, microbiological, and biomolecular exams were carried out. Histopathology showed systemic granulomatosis. Microbiological and biomolecular exams allowed us to identify the M. fortuitum as a main species. Finally, some considerations on the legal aspects of such disease are discussed.