So I have noticed that when I turn off the aquarium lights my Celebes Rainbow fish freak out, swim erratically and smack themselves into the tank walls in a frenzy. I'm always concerned they're going to hurt themselves.
I figured it must have something to do with the tank going from bright to dark so suddenly so I decided to try something. I first turn on the overhead light for the room my tanks are in before turning off the tank lights, and over a period of about 30-45 seconds, I slowly dim the overhead lights down to dark. Works like a charm! No more upset, stressed out fish at night time. I'm guessing doing this mimics sundown.
Try this if you have fish that also upset easily over the lights being turned off. :)