Clownfish

From Animal Crossing - Wiki on Neoseeker

A Clownfish in your pockets

Actual Clownfish

The Clownfish is one of the smallest saltwater fish in the game. It appears from April to the end of September. These many be small, but they do get 650 Bells when sold to Tom Nook! These will help new players accumulate bells when buying tools and other important starting items.