Dorado

The dorado (ドラド) is a golden fish. It is an extreme 100 centimeters long (40 in.), which is very long for a river fish. Its name, Dorado, reflects its color, as Dorado is Spanish for golden. This fish and the Stringfish are the most expensive river fish obtainable, selling for 15,000 Bells, beating the Arapaima and Arowana, who are both tied at 10,000 Bells. This fish is diurnal, meaning you will only see it during the daylight, or, more specifically, 4am - 9pm. It is the rarest summer river fish in the series. It will appear during late summer, from June to September. If you catch one once, it is very unlikely you'll catch another.

Donating to the Museum
When in the museum, this fish appears in the middle tank and the right tank.

In Wild World
Upon donating a dorado to the museum, Blathers will comment:

"Hoo my! Congratulations are in order! You've finally caught one, eh wot? Did the mighty dorado put up a good scrum? You've passed the test! You're initiated into the secret club of fisher folk!"

In City Folk
Upon donating a dorado to the museum, Blathers will comment:

"My hoo, I never dreamed someone in this town would hook a dorado! It's a fish that lives up to the image of El Dorado it conjures... I could gaze upon it all day!"

Further Information
Dorado is Spanish for golden, a suitable name for such a shining fish. In real life they are frequently confused with the Mahi-mahi, Coryphaeus hippurus (dolphinfish) also called the Dorado, which is the name of an oceanic fish that it is unrelated to and does not appear in Animal Crossing. The two real life species of "freshwater" dorado are Salminus maxillosus and Salminus hilarii. This fish can be found in the rivers of South America, most notably the Parana River of Argentina, and its tributaries.