Yeah dude, the following is for ALL NEWBS on piranhas:
You CAN NOT keep ANYTHING with a piranha. There are exceptions out there, such as plecos, tetras, etc, but chances are, they WILL be FOOD sooner or later.
Unless you want a school of Pygos (which even that can be dangerous/fatal at times), just about EVERY species of piranhas are solitary animals in the home aquarium. There are exceptions, such as the S. geryi, or S. spilo, but you're better off having them live alone.
If you CAN'T deal with the fact that they can't live with other tankmates, THEN DON'T GET A FREAKIN' PIRANHA.
END OF THE STORY.