First you need to understand whether the piranha is really an extremely aggressive creature that attacks everything that moves in the water. It may sound unexpected, but piranha is a very cautious fish, and poses no danger to humans. There is a large amount of evidence when a person swam in piranha-infested water without any harm to his health. This was fully demonstrated by Herbert Axeldorf, a famous biologist specializing in the study of tropical fish. To prove the safety of piranhas for humans, Herbert filled a small pool with piranhas and dived into it. Considered fearsome predators with an insatiable thirst for fresh flesh, piranhas are actually rather timid fish that dare not approach large creatures.
It is known that piranhas prefer to stay in large flocks, and if one piranha is seen in the water, there are always others nearby. But piranhas do this not because it is easier for a flock of predatory fish to fill up and kill a person who has entered the water, but because piranhas themselves are a link in the food chain for other larger fish species.
Moreover, experiments with piranhas have shown that, being alone, these fish do not feel as calm as if they were surrounded by other fish.