From Nookipedia, the Animal Crossing wiki
The Eel (ウナギ, Unagi) is a unique river fish in the Animal Crossing series. It is one of only two fish (the other being the Ribbon Eel) with a long and thin shadow. It is a rather expensive fish, selling for 2,000 Bells at Tom Nook's store. Like many Animal Crossing series fish, the Eel is a summer fish, and appears in the evening and night, between 4 p.m. and 9 a.m.
Donating to the museum
In Animal Crossing
"This is one of those fish that seem to mysteriously disappear from time to time. Drives fishermen batty, wot!"
In Wild World
The Eel can be found swimming close to the bottom and occasionally hiding behind the rocks in the river aquarium in the first fish room.
In City Folk
The Eel can be found in the right tank in the museum as a brown colored line moving across the bottom.
In New Leaf
"Eels are born in the ocean but swim upstream through rivers when they become adults. They do, however, eventually swim back to the ocean when it is time to lay eggs. Their blood can be toxic, which is why you've never heard of eel sashimi. It's just too dangerous. Cooked eel, however, is edible and has no poisonous effects to speak of, making it a popular dish. For a long time, the egg-laying locations and habits of eels were largely a mystery. However, they are gradually being revealed through research, making eels a very hot topic of study."
The American eel, Anguilla rostrata is a fish that spawns in salt water. After hatching, the baby eels make their way to North America, on the east coast. The eels, if hungry enough, may eat their own family to survive the trip to America. They prefer to hunt at night, which is referenced by the time it appears at in Animal Crossing.
Names in other languages