Could just be his colouring, it does look like a bruise doesnt it, I would keep an eye on it (excuse the pun) to see if it changes in shape or gets worse. Have you noticed him rubbing it on any of the tank decor.
hmm doesn't look like a bruise in the photo, i saw one in the store with only one eye mabye thats what happened to it. i wonder if its some kind of infection?
What's the difference between a fish and a piano?
You can't tuna fish!
