Finger lesion from cleaning fish tank?

    45 year old man with isolated, pale pink, firm papule on right 4th finger, increased in size over 1 week from 5 to 7mm in size. No fluctuancy or tenderness to palpation. Patient claims to have noticed lesion soon after cleaning out his saltwater fish tank (without gloves.)  The patient presents during the winter season in Boston.

    Stains for microorganisms are negative (AFB, GMS, Fite)