I have a lot of empty tanks I was thinking of uusin my 125 or 180Piranhas are fun to keep, but if you want an aggressive and active fish then I would look elsewhere. Most piranha are are shy and skittish. And most people are let down because piranhas aren't blood thirsty killers.
That being said they are cool fish and there are a lot of different species that you can keep. What size tank do you have?
In a 180 I would do an elongatus! Watch your fingers.