Arapaima

The Arapaima (ピラルク, Piraruku) is a river fish that appears in the summer. It appears between 4 PM and 9 AM. It is a very rare fish, making it the third most expensive river fish (10,000 Bells), after the Stringfish and Dorado and tied with the Arowana (which it is related to). Its shadow size is huge; similar to the Tuna and Stringfish, and likewise, it is the largest river fish. In the 'Bugs and Fish' window in, it is the last river fish. The fish is the second largest fish in the game, after the Shark at about 550 centimeters and it is tied for length with the Ocean Sunfish.

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Blathers will say this when an Arapaima is donated:

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Blathers' speech is as follows when the Arapaima is donated to the museum:

When donated, it is in the second tank in the first room.

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Blathers will give this speech upon donation:

This fish, when donated, will appear in the middle right tank.

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An information board in the aquarium will display this description about the Arapaima after donation:

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Once donated to the museum, the Arapaima can be found in the jungle tank in the first exhibit room sharing it with the Saddled Bichir and Arowana.

Real-world information
The arapaima is the largest freshwater fish in the world and it can be found in the Amazon River which is in South America. It is sometimes called the pirarucu or paiche. This fish can grow to about 3.2 metres, but normally about 2 to 2.5. The supposed maximum length is 4.5 metres, although this comes from an early 19th century second-hand report, and it has not been confirmed.The arapaima male is supposed to be a mouthbrooder, meaning the young are protected in its mouth until they are older. The female arapaima helps to protect the male and the young by circling them and fending off potential predators. Arapaimas breathe air and get their red coloring from their diet of red crusteceans.