What does Nemo call his smaller fin?

Nemo calls his smaller fin his “lucky fin.”

Nemo is born with one noticeably smaller, weaker fin, a condition connected in the story to the crack in his egg. Marlin worries that the fin will make Nemo vulnerable, but he and Nemo’s mother, Coral, frame it positively by calling it a lucky fin. The phrase becomes a warm reminder that Nemo should not be defined by a physical difference.

The fin is important to Nemo’s character and to the film’s themes. Marlin initially treats it as evidence that Nemo needs extra protection, while Nemo wants to prove that he can explore and act independently. During his time in the dentist’s fish tank, Nemo eventually uses his determination and swimming ability to help execute the escape plan.

“Baby fin,” “tiny fin,” and “magic fin” are plausible-sounding inventions, but they are not the film’s expression. The exact phrase is “lucky fin,” used by Marlin when discussing Nemo’s smaller fin. Nemo’s difference is presented as part of his identity, not as a reason he cannot accomplish difficult things.