From Nookipedia, the Animal Crossing wiki
Donation to the museum
In Animal Crossing
"Hoo hoo! Hullo, little fishy! Aren't we a tiny little fellow! Adorable! Why, I believe it stuck it's tongue out at me! Hoo-rumph! Nevertheless, we'll take care of the little blighter! I'll keep an eye on this mischief-maker myself!"
In Wild World
The Killifish can be found in the first aquarium tank after being donated.
In City Folk
Upon donating to the Museum in Animal Crossing: City Folk, Blathers will say the following:
It can be found in the left tube tank.
In New Leaf
Upon donation to the museum, the information board in front of the tank which houses the Killifish reads the following: "Killifish are small, travel in schools, and are known for having eyes higher up on their faces than others. They swim in shallow areas in rivers and mostly eat bugs that float on the surface of the water. This is why they have mouths as high up their faces as their eyes, making them effective hunters. Being the same temperature as the water they inhabit, getting grabbed by warm hands can shock them."
It is unclear what sort of killifish appears in Animal Crossing series. Judging by the size estimated within the built-in encyclopedia, it could be a Moonlight Medaka or "Japanese Rice Fish". However, all killifish have common traits, noticeably, their translucent skin which has made them an interest in biology due to the visibility of their digestive system. They are a harmless, common, aquarium fish.
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