My male ,Fizz, freaks whenever I turn the tank lights off but keep the room lights on. I guess it causes enough of a reflection that he thinks there's something there. He'll bump into the sides of the tank constantly like he thinks there's more to his tank than there is. Then he'll frantically swim all over his tank or he'll lay on the bottom of the tank right in front of his pineapple entrance and breathe heavily almost like he's hyperventilating. Poor thing
My female is in the tank right next to his and she doesn't do that when the tank lights go out, but her tank is shaped a little differently, so maybe hers doesn't cast the same kind of reflections. Hers is a 5 gallon and his is a 3 gallon (but he will be getting the same tank as her very, very soon).
Poor guy, I've just decided that I'll leave the tank lights on until I turn the room lights off. I really didn't want to have to do that since sometimes I stay up really late and didn't want them to have to be under the bright light for that long, but I guess I'm going to have to.