Designed by Akio Yamamoto, this puzzle belongs to a marine series of cast puzzle with usually 2 metal colors. As it's about marine sealife, the moves are not regular ones and can be very hard to discover and manipulate the pieces.
Actually this puzzle is not difficult and the moves are easy to discover: you will have to move and rotate the fishes.
I think this puzzle is a nice introduction of using rotations when solving a puzzle, and it's definitely easy to repeat. But still, it's fun and nice. Good puzzle for beginners....and for more seasoned who want to complete their collection :-)