I disagree with that...there has been a time when I didn't feed them for 3-4 days and everything was fine...if piranhas were such ruthless killers i'm sure the gourami, tetras, and raphael cat in there would show some signs of the p's going after them (nipped fins and what not).
One prime example..I did not feed them for I think about 3 or 4 days to see if I could get them to frenzy..anyway I had grabbed a bunch of feeders and moved the gourami and tetras from the p tank and put them all into a new tank I was cycling (I was getting impatient with waiting for a cycling tank) haha so then after these 6 days had gone by I threw a feeder into the tank, gone in a second (just a note I do not feed them feeders on a regular basis) so I put a second in and the same thing...so I say hrmm...put the gourami in nothing, put the tetras in nothing....put another feeder in....gourami chases it for a bit, the the p's eat the feeder. The p's always ignore them.