Sweetfish

The Sweetfish (アユ, Ayu) is a fish found in the river between 4 A.M. to 9 P.M. from July through to the first half of September, but is most common between July to August. It sells for 900 Bells in Tom Nook's Store.

In Wild World
"This fish is hardly sweet, contrary to its name. It still tastes... fishy. Indeed, if you expect some sort of lollipop in fish-form, you will be horrified!" —Blathers

The sweetfish can be found swimming near the surface in the pond aquarium in the first fish room.

In City Folk
"You can always tell where a sweetfish has been by the trail of bite marks it leaves behind, eh wot? The fish will scrape away at rocks that have food stuck to them, leaving jagged teeth marks. ...Quite wild behavior for such a refined-looking little fish, I should say!" —Blathers

The sweetfish can be found in the middle-right tank in the aquarium.

In New Leaf
"Sweetfish are migratory fish born upstream that travel to the ocean and then return to rivers to spawn. This migratory mind-set is what emphasizes their similarity to salmon. Adult sweetfish have a watermelon-like mossy smell when caught, since they eat moss on riverbed rocks. Sweetfish are very territorial and will attack rivals that encroach on their territory. Anglers use this combative nature against them by using lures made of other sweetfish."

Further information
The sweetfish, or ayu, is an amphidromous fish, the only species in the genus Plecoglossus and in family Plecoglossidae. It is a relative of the smelts and is placed in the order Osmeriformes. Native to the Palearctic ecozone, it is found in rivers, lakes, and coastal waters of western Hokkaidō in Japan southward to the Korean Peninsula, China, and Taiwan. The name "sweetfish" is from the sweetness of its flesh. Regarding its typical one-year life span, it is also known as nen-gyo ("year-fish"). It is Gunma prefecture's prefectural fish.