Bitterling

The Bitterling is a relatively common fish found in all series of Animal Crossing. It can be found in the river all day during the months of November to February. It sells for a reasonable 900 Bells at Tom Nook's store. In the 'Bugs and Fish' window on Animal Crossing: City Folk, the description reads; 'Easy to raise but tricky to breed, these are hard to find in the wild.'

In City Folk
Blathers will tell you 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.

Further Information
The Rhodeus sericeus or 'Bitterling' is a harmless fish that lives in ponds, lakes, marshes, muddy and sandy pools aswell 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).