Pop-eyed goldfish

The Popeyed Goldfish (デメキン, Demekin) is a rare fish in the found in the river. It sells for 1,300 Bells in Tom Nook's Store or Re-Tail and appears all year from 9 a.m. to 4 p.m.. Its shadow size is tiny. The flavor text from the and  versions refers to the cartoon Popeye, where spinach is a source of strength.

In Animal Crossing
As a Popeyed 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
When a popeyed goldfish is donated, Blathers will say this:

"You should know, the eyes on the popeyed goldfish can occasionally pop out! There's nothing wrong, of course, and if you let the fish be, they go back in. Even so, the first time I witnessed it, MY eyes nearly popped out, wot wot!"

In City Folk
When accepting the fish for the Museum, Blathers will say this:

"Popeyed goldfish look like any other ordinary goldfish whilst still juveniles. As they begin to mature, their eyes bulge outward and gradually assume that distinctive appearance. Imagine how appalled you'd be if you brought one home thinking it was a normal goldfish...".

In New Leaf
An information board in the aquarium will display this description of the Popeyed Goldfish upon donation:

"They're known for their protruding eyes, as the name indicates, but this feature is actually a detriment. Their vision is quite poor, to the point that they will swim past food and frequently bump into things. They are a type of goldfish and get their distinguishing eye pop about two months after birth."

Further information
The Carassius auratus shares its scientific name with all types of Goldfish. The Popeyed Goldfish, or Black Moor Goldfish, is a harmless fish that lives in slow-flowing ponds, ditches, rivers, and lakes. It is a specially bred type of carp with large, protruding eyes, which, to some people's surprise, has poor eyesight as a result of its heavy inbreeding. It is related to other types of telescope fish, Carp and is indirectly related to the Koi. It feeds off of small insects, crustaceans, and plants. In China, it is called the dragon-eye. They are good for people who are just starting their freshwater tank.