My roomate's Exodon paradoxus seems to be missing a chunk of skin. Sorry the picture isn't the best but please tell me what it is and what the treatment might be.
Not sure if you've ever heard of lepidophagy, interesting behaviour and wonder if it's related.
http://en.wikipedia.org/wiki/Lepidophagy (http://en.wikipedia.org/wiki/Lepidophagy)
If he has multiple exodons, they are very nippy and known for taking chunks out of each other...