I'm not sure specifically which 'nemo sad story' you're referring to. It could be about a fish named Nemo having some sort of misfortune like getting lost, separated from his family, or facing danger in the ocean.
Finding Nemo is purely fictional. The characters and their adventures are made up to entertain and charm audiences, rather than being drawn from real-life events.
The story begins when Nemo is taken away from the ocean. Marlin, overcome with fear and worry, sets out on a dangerous adventure to rescue him. Along the way, he meets various characters and faces numerous challenges.
Finding Nemo is about a clownfish named Marlin who goes on an epic adventure to find his son Nemo after he's captured and taken to a dentist's aquarium. It's a tale of love, courage, and the lengths a parent will go to for their child.