Fish tank granuloma is a rare skin infection caused by Mycobacterium marinum. It occurs after exposure of skin abrasions to contaminated water or infected fish. The majority of M. marinum infections today are fish tank-related. The most common presentation is a solitary nodule, often with sporotrichoid spread. Other presentations do not occur often. The diagnosis is often delayed because of lack of suspicion, nonspecific histopathological findings, and frequently unsuccessful cultivation. Here we present the case of a 37-year-old male with M. marinum skin infection, presenting as erythematous scaling plaques. Because the initial results of laboratory and histopathological examinations were negative for a fungal infection or nontuberculous mycobacteria, the patient was treated empirically with several systemic antibiotics and antifungals without any success. Finally, the diagnosis of fish tank granuloma was confirmed 3 months after the initial presentation of the patient. After the introduction of treatment with rifampicin and clarithromycin, complete clinical remission was observed after 6 months of therapy.