I have 7 Green Chromis and they disappear at lights out. In fact, about an hour before the timer turns off the lights, they start looking into caves and jockeying for position. It is the only time that they show any kind of agressiveness. In the morning, when the lights come on, they start appearing, one by one.
The other thing that I've noticed about them is that during the day, with my office lights on, they kind of scatter around the entire tank, checking the water column for food. When I turn off my office lights, they immediately make a dash for the center of the tank which is lit by 70 watts of NO florescents over the center of tank. Each end of the tank is pretty dark. When I turn on the office lights, they spread out around the tank again.
I think that they are afraid of the dark :scared:
Here's what it looks like with the office lights out:
