Just yesterday as I was feeding him I noticed that on his front leg he had a large, purple/red sore spot! The only thing I can think must have happened is he bit and grabbed on hard enough to hurt himself (I made a post over a year ago about his arm biting behavior, never quite figured out what that was about..). I haven't seen him biting himself lately but I guess it happened when I wasn't around. He was not bleeding.

Here's the best picture I could get, he wouldn't let me touch it or pull his arm out to get a better look.
I'm confused on why he's doing this...I've also noticed him biting on the plastic intake of the filter. Is he bored or agitated by something? I keep the water very clean... has anyone ever seen anything like this before or have suggestions on what I should do to help him?

