This video says about itself:
Swimming With Red Belly Piranhas – River Monsters
29 April 2015
Jeremy has a pool filled with 100 Red Belly Piranhas, to demonstrate how they will only attack when blood and flesh are thrown into the water.
The video shows something different from the traditional view on piranhas.
The animals grow up to 20 – 30 cm.
LONDON – Despite their fearsome reputation, piranhas are wimps that gather in large shoals to protect themselves from predators, scientists said on Monday.
“Previously it was thought piranhas shoaled as it enabled them to form a cooperative hunting group. However, we have found that it is primarily a defensive behavior,” she said.
“Their cautious behavior is crucial to avoid being eaten,” Magurran said.
Her work with the Mamiraua Institute in Brazil shows how shoal sizes increase in relation to predation risk, especially when water levels in the Amazon basin are low, giving piranhas less room to escape attack.
The research is featured at the Royal Society‘s summer science exhibition in London.
See also here.
Piranha evolution: here.