So-called fish tank or swimming pool granuloma in the back of the hand. The erythematous painless nodule may also ulcerate. The granuloma is caused by the bacterium Mycobacterium marinum, which lives in warm waters like in aquariums. It can penetrate only skin that is already broken. In the case presented in the picture, the infection has probably occurred through the broken nail wall when the patient has worked with his fish tank. From there, the bacterium has found its way to the back of the hand through lymphatic vessels. The mycobacterium does not, however, cause a systemic disease. A long course of antibiotics is required for cure.