From Nookipedia, the Animal Crossing wiki
The Goldfish (キンギョ, Kingyo) is one of the smallest fish. Despite its tiny size, it is actually quite rare. It can be found in the river. It has a tiny shadow and is 15 centimeters long. It sells for the relatively high price of 1,300 Bells. For most fish, when placed inside the player's house, they appear in a fish tank. This does not apply to a Goldfish; they swim in a blue-rimmed bowl with seaweed.
Donation to the museum
In Animal Crossing
As a Goldfish is donated, Blathers will say this:
"Those adorable little fins! Those huge, googly eyes! Even I am helpless in the face of their cuteness! We will take special care of this precious fellow. Welcome to your new home, you darling little fishy!"
In Wild World
This is Blathers's speech about the newly donated fish:
"Hoo, in my youth, I once had a goldfish in my room. His name was Sir Blathers. Goldfish are actually quite sociable creatures, eh wot? At least they are when they're being fed, that is. Who can resist them?"
It appears in the first tank in the aquarium.
In City Folk
Blathers will say this when handed the fish in the museum:
"Hoo now... Do you know how large goldfish can get? Well, they can grow up to... ONE FOOT (30 centimeters) LONG! The growth of a goldfish in a tank is inhibited by tank size, so you rarely see them that big, I'd imagine. Even so, our tanks here at the museum are spacious... so expect our new goldfish to make a BIG splash! ...Hoo now? Not at all funny, you say?"
It can be found in the small tube-tank in the aquarium.
In New Leaf
Upon donation to the museum, the information board in front of the tank that houses the Goldfish says the following:
"These are aquarium fish that came about by selectively breeding types of crucian carp. It took hundreds of years to get the long tail fin and beautiful color of the current breed. Even red goldfish are black when they are young and still retain other vestiges of the crucian carp. They turn red as they grow, thus gaining their much more commonly known look."
This goldfish is the Fantail breed. It has two fins instead of one. It has an egg shaped body, a high dorsal fin, but no shoulder hump. It is the western variation of the Ryukin breed. They do not like cold water. These, like all goldfish, are commonly kept as pets, and prefer a water temperature of 13 - 21 degrees Celsius/Centigrade.
Names in other languages