Difficult to say post-mortem, but that does look like an injury. If the puffer is largish, I would suspect that...they definitely can change they behavior over time. Is the only other issue the skeletor being off feed? Eels do that from time to time, sometimes it isn't an issue at all.
One thing I want to clarify - stray voltage is a human health issue (shocks) if it gets bad enough, but will not harm the fish as they are not grounded. For decades, this has been implicated in all sorts of aquarium issues, but just as many tanks have issues and don't have stray voltage. Much of what is measured is induced voltage from pumps. Sometimes it is from a bad seal where the cords enter a powerhead. My father was an electrical engineer and he explained it to me this way: birds can perch on a high tension line with no problem because there is no electrical potential to ground. If their wing touches the tower, they would explode...
Jay