When I search fishbase for "red bellied pacu" I get the scientific name I posted above. It is only when I shorten it to "pacu" that I find it labelled as Piaractus brachypomus.
I think that there must be multiple different species of pacu and the one fishbase knows as "red bellied pacu" is called Colossoma macropomum.
The species Inka posted a link to does not seem to have the common name of "red bellied pacu" which leads me to believe we are posting about different species altogether.
I was unable to get the search results links to work but just search fishbase.org under common names for both red bellied pacu and pacu to see what I mean.