first off, welcome... piranhas are pretty skittish fish, but they will eventually become comfortable with their surroundings. if you got your fish recently, then you just need to give them a bit of time. some people may suggest dimming the lights or not having any at all, but in my experience, the best thing that you can do is to put your lights on a timer so that they go on and off at the same time everyday and let them get used to it. i have normal lighting over most of my tanks and bright lighting over my mixed shoal planted tank and i don't have any problems with my fish being scared whatsoever, they are all very active and don't run for cover when people go near their tanks. just make sure you provide a bunch of cover for them and they'll come out of their shells.