Years ago, I had what looked like a "Red Belly Piranha," only this fish got twice the size in the half the time and was just a gentle giant.
Thanks as always,
They get up to a foot long, hope the link helps.
Here is a link that might be useful: Red Piranha profile
Probably a red pacu.
Here is a link that might be useful: Red Pacu description
Try putting your finger into the tank and see if it will destroy your finger, so you will know if its a deadly Piranha or a peaceful Pacu. Dropping a beta into the tank does not answer your question, since large fishes consider betas a glorious snack.