The piranha tank is an apparatus designed for carrying out executions. The device consists of a foot pedal trigger and a collapsible bridge suspended over a tank containing predatory fish. The equipment was featured in EON Productions' 1965 James Bond film You Only Live Twice.
Blofeld's Piranhas are predatory fish that Blofeld keeps in a pool in his Volcano Lair. He uses them as a method of execution for those who displease him. The fishes can shred a human to the bones in less than 30 seconds.
The only unlucky person who is thrown in this tank by Blofeld is Helga Brandt. She was a sexy young woman working as a low-rank assistant and secretary who was ordered by her superior Osato to kill Bond, but she had sex with him first and finally failed to kill him. She was then called by Blofeld but was unable to justify herself when Osato put the whole blame on her. Consequently, Blofeld, who doesn't tolerate any failure from incompetent assistants, decides to kill her with that very cruel way and the unfortunate girl is dropped into the tank. The terrified secretary vainly calls for help but gets quickly devoured alive by the piranhas, screaming with pain as the fishes tear her whole body to the bones until she definitely disappears under the water. Her horrible death basically serves as an example for the other SPECTRE employees and shows that Blofeld doesn't have an ounce of leniency for them and doesn't care to kill women and young employees. Brandt was a loyal henchwoman who assumed all the dirty work to satisfy her superiors, but unfortunately for her she failed one task and painfully paid it with the piranhas, showing the employees what can happen to them if they fail in their job even once.
Bond later knocks Blofeld's bodyguard Hans into the pool and jokes "Bon Appetit" to the Piranhas.