Pale chub

The pale chub (オイカワ, Oikawa) is a common fish found in the river in all of the games. It can be found in the river between 9 a.m.-4 p.m. all year round. It is an inexpensive fish that sells for 200 Bells at Tom Nook's store.

Donating to the museum
When handing the pale chub Blathers in the Museum, he will say the following before thanking the player for the donation:

In Animal Crossing
"Oh? Is that a fact? It's sometimes called a mountain trout? But it's not a trout! What rot! Well, regardless, I'll see that it gets the proper care. You have my word on it!"

In Wild World
"You know, I'm sure these are technically edible... Even so, I don't think there are too many people who actually DO eat them, wot? I suppose if one were to batter them with some delectable spices..."

In City Folk
"The pale chub is a freshwater fish. During its spawning season, it turns a vivid blue, wot wot? But all the eggs do have similarities... Telling what sort of beastie laid the egg is difficult, wot wot? Rather fascinating how a fish normally so plain can put forth such an effort when love is on the line... Perhaps I might stand to adopt that fish philosophy myself, and quite soon!"

In New Leaf
Upon donation to the museum, the information board in front of the tank says:

"These fish are related to carp, with distinguishing features such as long ventral fins and protruding mouths. They have good jumping abilities, which allow them to jump up and grab bugs hanging around the water. They are known as very active fish with rather scrappy attitudes, making them tenacious survivors. During mating season, the male's stomach turns pink and its back turns blue. Females love this."

Further information
The pale chub is type of freshwater minnow. It is listed as harmless and not in any danger. It can be found in Korea, where its real name is the Pirami. It lives in fast flowing rivers and streams, and it feeds off of insect larvae and algae, and smaller fish. During mating season, the male Pirani changes color rapidly to impress a suitable female. They actively feed during the evening or when the weather is overcast.