I'm wondering if an evolution tree would help show the player what they are striving after. It could be a menu that shows the unlocked evolutions as images of each fish, then either darken an outline of the next evolution or have a small white node indicating that there is more to do with the last unlocked fish. I'm probably not explaining it very well, but the best example can think of is the game Cells to Singularity: Evolution. Its a pretty popular clicker game with multiple branching evolution trees. The game also features little environments with the related unlocked animals/plants/things. You can even click on the items in those little scenes and the cam follows that clicked object similar to the way yours is implemented.
There is an evolution tree on the right side when you click on a fish. It wouldn't have shown up for you until toward the end of the Tutorial so maybe you didn't make it that far.
I would like to make a larger version of it in its own menu and I agree it helps the player understand what they are working toward. I like your suggestions about how it could work.