If they are selling it for $20 at your lfs, I'm 99% percent confident that it is a rbPacu. Pacus and oscars just seem lame to me imo...
Also probably is a pacu as a red belly piranha would not reach 10 inches in less than optimal conditions which I'm confident exist at the LFS.
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I would steer away from that fish if I were you and I would stay away from this guy at the lfs...he might tricking you to buy the pacu.