From Nookipedia, the Animal Crossing wiki
The barbel steed (ニゴイ, Nigoi) is a relatively common grey fish found in all versions of Animal Crossing series. It is found in the river all day and all year round. It is an inexpensive fish, as it sells for 200 Bells. Its shadow size is between small-medium, the same size as a Carp.
Donating to the Museum
In Animal Crossing
Blathers will give this speech upon donation:
"Even run-of-the-mill, common fish are valuable resources, you see. All part of the grand tapestry, wot! We will take excellent care of this fellow, oh yes. You can rest assured. Excellent care, indeed."
In Wild World
When the barbel steed is given to Blathers, he will comment:
"This might look like a carp, but I heard it doesn't taste like one, eh wot? Indeed, I should think this fish is more suited to catching than to eating."
It can be found in the first tank.
In City Folk
Blathers will say this upon donation:
"While barbel steeds have many of the same features as carp, they're a bit less exciting, to my mind. For example, both have whiskers, wot? Yet the barbel steed's are just a hair shorter... It's rather like a fan doing an imiation of his or her favorite actor, or something to that effect."
It can be found in the middle-left tank in the aquarium, in the museum.
In New Leaf
An information board in the aquarium will display a description of the barbel steed after donation.
"Barbel steeds have a longer distance between their eyes and mouths than carp do. This makes them look almost like they're averting their eyes downward in embarrassment. Their whiskers aren't as long as carp whiskers, so the resemblance is still only minor. They are full of bones, but they're edible nonetheless and actually taste similar to flounder."
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the barbel steed has been caught, the following information can be found in the fish menu:
The Hemibarbus labeo or 'Barbel Steed' is a harmless fish that lives in the rocky, fast flowing bottoms of rivers. It is carp-like in appearance. The name 'barbel' is derived from the Latin Barba, meaning beard which is a reference to the two pairs of barbs by its mouth. There are different types of Barbels scattered across the world- they can be found in Europe, Asia and Africa. The Barbel Steed however, is an Asian fish, found in the Amur basin where it is locally refered to as the Amur barbel (Barbel Steed).
More information on this topic is available at Wikipedia.
Names in other languages