How old are your piranhas? How long have you had them? Do you have any live plants?? If so, this might not be helpful for your plants.
In my situation, I just keep the light on for about 8-10 hrs. At first they did not like it, but later got use to it. Right now they seem not to mind, however they just stay still as the swim in the power head. Ater I turn the lights out they are super active!!
Just make sure the lights are not overly bright. You can solve this by candy cane the lightbulb with foil. or even spray painting one side on the lightbulb. I believe there is something pinned about this in the tank and accessories forum.
Good Luck