From Nookipedia, the Animal Crossing wiki
The Dab (カレイ, Karei) is a fish in the flatfish group. It has the least letters in its name of all fish, except the Ray, which has three as well. Coincidentally, they are both in the flatfish group. It can be found in the ocean. It, when sold at Tom Nook's Store, will get the player only 300 Bells. The lighter shaded Olive Flounder is commonly mistaken for the Dab in real life, as well as by the player in Animal Crossing: Wild World and Animal Crossing: New Leaf when they catch one.
Donating to the Museum
As with all fish and bugs in Animal Crossing series, the Dab can be donated to the Museum by talking to Blathers, the curator. When donating, Blathers will tell the player a little bit about the donation. Below is what he has to say in each game.
In City Folk
"By the by... Just so you know, eyes on a dab are positioned on the right, but on the left on a flounder. When young, these fish are shaped like ordinary fish, but... As they mature, they start becoming flatter in preparation for adulthood! What professionalism! You can tell this is a very responsible fish, wot?"
The Dab can be found in the tank at the back of the room, with other sea fish.
They do not show up when olive flounders are around.
The common dab is the smallest flatfish in the world. It can be found on sandy seabeds, most commonly in the North Sea off the east coast of England.