My shoal has been on the same time cycle for 8 months, it will flick on at 8am and flick off at 7pm and they never move or get scared, IME if their resting and the room is still and u randomly fire up the lite they will get skiddish cause they don't know what's going on, once they realise they will calm down. As Baller suggested u need to get them on a perfectly timed night and day cycle then they will get used to it! Piranhas are the most skittish fish I've ever had, they are very sensitive to change so they freak very easy, once they settle in ur tank they will seem like different fish, but it takes time, once u have their trust they will greet u!
This also could mean ur lites too bright, like I've suggested to u before Bw may help with this or Atleast make them more comfortable. Just try and leave everything alone so that they have time to adjust, I know u wanna move stuff around and mess with the tank, but let them get adjusted especially since they are young. Some keepers don't use lites at all. And don't forget bed time stories, that is a big thing with my shoal!
Go S. Vettel #1 rb8
This also could mean ur lites too bright, like I've suggested to u before Bw may help with this or Atleast make them more comfortable. Just try and leave everything alone so that they have time to adjust, I know u wanna move stuff around and mess with the tank, but let them get adjusted especially since they are young. Some keepers don't use lites at all. And don't forget bed time stories, that is a big thing with my shoal!
Go S. Vettel #1 rb8