[NanoZoas]

I am new to this marine aquarium hobby, and have a small tank, that I let run for two weeks before adding a test fish, a yellow finned damsel. On the first night I noticed it had a wound on its left gill area, I thought maybe trauma during the capture at the store. The tissue is white and the blue scales are gone in that area. It has not gone away almost a week later and now the eye and tissue around the eye is black. The fish is not as active as it was when I first got it, though it still eats great. I did not intend on keeping the fish, but I didn't want it to die! What can I do? Is this an infectious disease that now my take has?