Introduction:
Fish tank granuloma is a rare dermatitis caused by Mycobacterium marinum. Infection occurs through contact of damaged skin with water or aquatic animals infected with these bacteria.

Aim:
It is likely that skin infection caused by M. marinum transmitted from Clarias gariepinus has as yet not been reported in literature. Our case report is presumably the first publication. Dermatoscopy as a useful method in the diagnosis of skin infections caused by atypical mycobacteria. Our article presents new dermatoscopic features of fish tank granuloma.

Case study:
The case report applies to a 30-year old, white, Caucasian man referred because of a single, well-demarcated plaque measuring 2 × 4 cm, localized on third finger on left hand above proximal phalanx, without subjective symptoms. The diagnosis was confirmed by medical history, dermoscopy and histopathological examination. Patient was successfully treated with sulfamethoxazole and trimethoprim.

Results and discussion:
Due to the fact that M. marinum infection is uncommon, in case of clinical suspicion it is necessary to perform skin biopsy, culture and tissue PCR analysis. Knowledge about the opportunities and limitations of theses laboratory tests is pivotal to reasonable clinical decision-making.

Conclusions:
Dermoscopy is useful to make a diagnosis, but there are still too few accounts in literature. The correct diagnosis determines the effective treatment.