Is the tank in a high traffic area of the house? (Could be stressed out)
Are there any perches? (My IJ, even at 3 years old is still pretty much exclusively arboreal, so is my baby Jag)
Is he off feed?
Maybe he's in shed? (My IJ's shed process is like a month long, and she is either hiding or soaking the entire time)

And it's very possible that he just likes it in there.