Red belly piranha or red belly pacu?

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Body shape is completely wrong for a baby piranha. It's too long and the fins are too large for a baby piranha. Even from the poor pictures, the mouth is also pretty obvious
 
Feeding it meat wouldn't help. Pacu will eat anything. Whether it's a piranha or pacu, it would probably at least try to eat it. It is however DEFINITELY a pacu.
 
There is no way to tell by feeding it. There's no reason to do anything to figure it out because you've already been told
 
Looks like a pacu to me. The corner of a pacu mouth is rectangular shaped, while a piranha corner of the mouth is an acute angle. Piranhas have a prognathic mandible--their jaws jut out. Pacus do not have a prominent lower jaw.
 
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