Fish banging in tank

[Verna Gross]

My Talipia bang into the walls of the IBC tote at night. It is really loud. I have only witnessed this after the sun sets and the tank room us dark. My fish are inside any ideas? Might I have a bully in the tank that only gets mean in the dark? I have 36 Talipia and 3 goldfish. All are about 8 mos old