The two answer words to this flat differ by a single letter.

In a happy-ending story, we picture
Nemo and Dory as our _ _ _ _ _ _ _ .
But it's different with the piranha,
Where tales don't end the same.
Then people see _ _ _ _ _ _ in the streams
Where humans are fish fair game.

Answers:

FRIENDS
FIENDS

Credit:
The Enigma  
NPL Magazine
April, 2019
COUSIN JOE (adapted)