From Nookipedia, the Animal Crossing wiki
The Piranha (ピラニア, Pirania) is a rare fish that appears in the river during the day time in the summer. It is quite expensive for such a small fish; in fact, it sells for 2,500 Bells in City Folk, making it the most expensive 'small' fish.
Donating to the Museum
In the museum, the Piranha appears in the bottom right tank (Animal Crossing), middle tank (Wild World), or the right tank (City Folk), where it will bash against the walls of the tank in an attempt to reach the player, although this depiction of piranha is inaccurate.
In Animal Crossing
"Dear me! My mind boggles! I had no inkling that we had such frightfully dangerous fish in these waters! I suppose that would indicate they're not as lethal as they are made out to be... Or are they trying to lull us? Have no fear! We'll look after your sharp-toothed friend. For everyone's sake!"
In Wild World
"Piranha are rather bony. I can't say I've heard good things about their flavor. Some restaurants do serve them, of course, but as a tourist attraction, eh wot? These things probably sit about discussing OUR flavor, you know."
In City Folk
"Most people see piranhas as savage fish that enjoy tearing prey apart with their teeth... However, they're actually quite timid, wot wot? They're often treated like bad guys due to their scary faces. I hope we haven't hurt their feelings..."
In New Leaf
"They have terrifyingly sharp teeth, which contributes to their ferocious reputation. Oddly enough, they are actually quite timid, though they are still very dangerous fish. If they can't find food, they will resort to cannibalism or tear apart any animal they can grab hold of. Take care when catching them so they don't end up biting your hand with their sharp teeth!"
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the Piranha has been caught, the following information can be found in the fish menu:
Giving to Wendell
The red-bellied piranha has a reputation one of the most ferocious fish, but this is a widespread myth perpetuated by the media. In reality, they are scavengers that are easily scared. Live-feedings for captive piranha are also unhealthy for them, which again, is falsified in the media. They can be aggressive, but only when they are deprived of food. Red-bellied piranhas can be found in the Amazon River. They grow to about 33 cm and weigh 3 1/2 kilograms.
Names in other languages