Don't worry, not gravel or anything. I have a huge rock in there. I came home from work today and there was what looked like sand just piled high in the corners where the filter apperantly doesn't get to that well :p I feed him plenty (far far more than I ever should I recently learned) and there is always some cuttle bone in there for him. I really can't imagine that the sand is coming from anywhere else as he lives the stark life and theres nowhere else for it to come.
Does he need a freaking chew toy in there? I have cut down on his food since learning I was excessivly over feeding him. Could that be a possibility as to why he seems to be gnawing on it?
Granted I have never actually seen him... but as I said, I just can't see it coming from anywhere else.


