Yellow perch

The Yellow Perch (イエローパーチ, Ierōpāchi) is a fish found in the river at any time of the day between October and March. Its shadow size is between small/medium, roughly the same size as the Rainbow Trout and Carp.

Donating to the museum
The Yellow Perch can be donated to the museum by talking to Blathers, who will give a small speech about the donation:

In
The Yellow Perch appears in the middle-right tank of the aquarium.

In
The information board in the museum says the following:

The Yellow Perch can be found in the larger of the tanks along the north wall of the freshwater fish room.

Real-world information
The yellow perch is a species of perch found in the United States and Canada, where it is often referred to as the shortform perch. Yellow perch resembles the European perch, but are paler and more yellowish, with less red in the fins. They have an active lifestyle during the day, when they feed. They have 6-8 dark vertical bars on their sides to protect themselves from predators. Yellow Perch are fairly easy to catch and are often caught while fishing for species in which they share the same body of water. As mentioned in the 'Bugs and Fish' window in City Folk, there are lures designed to look like perch, usually to catch bass but other fish are attracted to them too, such as Pike. Yellow perch weigh about 1.8 kg (4 lbs) and generally 10 to 38 cm (4 to 15 inches) long.