I have not had any problems with piraibas starting to feed, when they are comfortable.
The bigger one from Wes presented an irregular feeding pattern but this was because I kept the 18" fish with 2'-4' barbels and extensions in a measly 240 8x2x2 until around 20"-22", when it got too skittish.
But all in all, I didn't have to do anything special to get them to start on my staple of catfish farm pellet and frozen marine bait fish.