From Nookipedia, the Animal Crossing wiki
The Butterflyfish (チョウチョウウオ, Chōchō̄o) is a small, yellow fish that appears in the ocean during April to September. It is uncommon. It can be found all day long. This fish will sell for 1,000 Bells, and must not be confused with the Sea Butterfly, which also sells for 1,000 Bells, but appears in winter. The Butterflyfish appears in Animal Crossing: City Folk and Animal Crossing: New Leaf.
Donation to the Museum
In City Folk
"This is a butterflyfish, but the truth is that there are over 100 varieties of this creature... They live in the oceans of the far south, and each variety has its own distinctive coloration. These fish are most often seen socializing in groups among stalks of coral. Rather chatty, wot?"
It can be found in the back tank with the other ocean fish.
In New Leaf
Upon donation to the museum, the information board in front of the tank that houses the Butterflyfish says:
"Butterfly fish are considered very cute pets, popular because of their beautiful nature. They glide through the warm seas by flapping their yellow bodies as if they were butterflies. Their diets are surprisingly hardy, as they gobble coral and poisonous anemones using narrow mouths. Butterfly-fish couples have been known to live together for 10 years or more. Often they can be seen swimming around in pairs. You'll likely never find a better underwater romance."
Chip will say this when given a Butterfly Fish:
This butterflyfish is a Yellow Longnose Butterflyfish (Forcipiger longirostris). In the game, it does not have the long nose. There are many species of butterflyfish, some of which bear a resemblance to the barred knifejaw.
Names in other languages