This might be a long shot, but you mentioned you feed him fish. What kind of fish, and where do you buy them from?

I have read a couple of things online, but it could all be a load of bull, but that feeding feeder goldfish to those small colubrid and water snakes is generally not a good idea because of something the goldfish carry.... I would have to do a search to refresh my memory.

Like I said, long shot.