From Nookipedia, the Animal Crossing wiki
The Bitterling (タナゴ, Tanago) is a relatively common fish found in all games of the Animal Crossing series. It can be found in the river all day during the months of November to February. It sells for a reasonable 900 Bells, or 1,300 Bells in Doubutsu no Mori and Animal Crossing, at Tom Nook's store.
Donating to the Museum
In Animal Crossing
"Even run-of-the-mill, common fish are valuable resources, you see. All part of the grand tapestry, wot!
In Wild World
Blathers will say this as the fish is donated:
"I must say, I've never personally considered eating one of these before. It's so very...goldfishy... No, not particularly tasty-looking, to be sure..."
After donation, the Bitterling can be found in the front tank in the first room.
In City Folk
Blathers will say this when handed the small fish:
"It's quite common for enthusiasts to keep bitterlings in the same tank as tropical fish, eh wot? Indeed, once upon a time, this fish was a popular choice for the private fishing ponds of the rich!"
It can be found swimming aound in the middle-left tank in the Aquarium after donation.
In New Leaf
After donating the Bitterling, the following information will be displayed by its tank:
These fish are so small, they can fit their entire body in the palm of your hand. During mating season, the male's belly turns a light orange in order to properly attract a mate. They live in rivers and lakes and deposit eggs in large bivalves where the eggs hatch and live for a while.
The Bitterling can be found in the larger of the two left-hand tanks in the freshwater fish room.
The Rhodeus sericeus, or "Bitterling" is a harmless fish that lives in ponds, lakes, marshes, muddy and sandy pools, as well as the backwaters of rivers. It feeds mainly on plant material, but has been known to feed on small insect larvae. The Bitterling lays its eggs in the mantle cavity of mussels. The bitterlings can be found in central and eastern Europe, and are often seen in public aquariums and can be bought for private aquariums. It is a low-risk fish (not under threat).
More information on this topic is available at Wikipedia.