Personally I think he's just scared atm. During a water change or if I'm doing anything in general where my aro feels trapped, he'll sit at the bottom and not move, which is good because I wouldn't want him to jump during the water change, lol. But um, my aro usually gets over it by the time I'm done. I'll leave and turn off the lights and back up he goes. They get spooked real easy, especially at that size. Maybe someone tapped the glass or one of your pets hit the tank?