Moray eel

The moray eel is a fish you catch in the sea. Look for a long, thin shadow. In the northern hemisphere look for it in Aug – Oct, at any time of day. It can only appear after you have caught 20 fish. Nook's Cranny pays 2,000 Bells, C.J. pays 3,000 Bells.

Look for a long, thin shadow in the sea once you have 20 catches. Only it and the ribbon eel make that shape, so it is easy to pick out, and it sells for 2,000 Bells.