I agree with ldk -- its most likely stress. When I had my discus tanks running, I had to sneak in the room and hide behind a chair just to see the discus! Dither fish encouraged them to come out. However once I did a partial water change, they were back to hiding.
Leave a light on in the room to prevent them from freaking out when you turn off the tank lights.
Lastly, pay close attention to their stomachs. Make sure they don't go on any hunger strikes. First sign of trouble is a pinched off gut.