why do you think the op's fish is a capapretum? because it still has spots? Juvie piraiba usually don't lose their spots until the reach sexual maturity. I would agree however that yours is a capapretum, the maxiallary barbels do not extend past the adipose fin which is a feature of filamentosum as described by John Lundberg the person who differintiated the two fish as separate species read the link posted by chub-by for further information
The whisker you see was partially bit off by a tankmate several weeks ago...on the other side it goes almost to the tail.
These two are the same fish and about the same size...can't you tell?