Fish is Fish by Leo Lionni

Two best friends, a minnow and a tadpole, are practically inseparable until the tadpole grows legs and decides to explore the world beyond the pond. When the tadpole, now a frog, returns to tell his friend of the extraordinary things he’s seen, the minnow, now a fish, tries to follow in his footsteps, but quickly finds that land is not what he expected. Friendship truly saves the day in this imaginative tale of a fish out of water.