Red Belly Pacu or Red Belly Piranha ?

Hi community !
I am wonder if they call Red Belly Pacu or Red Belly Piranha ? Which one is more appropiate? I have no clues what's the different between a Pacu and a Piranha.

I bought a Red Belly Pacu for my son ( that's what they called in the Local Fish Shop ). He's little around 3 inches now. My son put that "Pacu" together with his Parrot Fish in a 180 gals tank. The question here is , will the "Pacu" harm his Blood Parrot when that "Pacu" get big ? He's also has some other fish in the tank such as arowana, etc..

Thanks again and happy holiday.

Thanks alot for your reply Big Roc. So as what you said, my red belly Pacu is a Pacu or a Piranha ? If it's a Piranha, is it more aggressive and dangerous because of its teeth ? He's only almost 3 inches now but I saw him chase my son's 8 inches long Tinfoil Barb.
Please advice me, thanks again Big Roc

Here are some pictures of this fish,

So they're only cute when they're young

I will definitely get rid off him tomorrow, either return him to the fish shop or just give him away to the neighborhood boys whoever wants it. I can't afford another big tank for my son just for him to grow this monster later on, or my son doesn't want to see his other fish be eaten by this Mr Pacu either.

I am still seeing him chasing other fish in the tank now, what a Mr Pacu . Thanks again everyone.