I've had a number of Pacu's in my 300 tank. Yes some of them go crazy when the lights go off. It seems that my Pacu only had problems with other Pacu. I put two in at the same time and they grow up together. When they got over a 12 - 14 inches they started fighting and splashing water everywhere but they didnt pick on any other fish. Pacu's can be very paceful fish but they will get large and eat anything they want. I think they are like any other fish some are Good, Some are Bad and Some are Ugly. 
