Rainbowfish

The rainbowfish is a fish you catch in rivers. Look for a tiny shadow. In the northern hemisphere look for it in May – Oct, from 9 AM – 4 PM. Nook's Cranny pays 800 Bells, C.J. pays 1,200 Bells.

An X-Small river shadow that bites in the middle of the day. Despite its bright looks it only sells for 800 Bells, so catch one for the museum and move on.