It’s the details that stand out whenever a classic film is converted to 3D.

With “Finding Nemo,” the shimmering sea surface, scratches on the lens of a diver’s goggles, and smudge marks Nemo the clownfish makes when he mashes his face up against the glass wall of the aquarium that imprisons him all pop off the screen in the 3D reissue of Pixar’s undisputed masterpiece.

The fish seem to float between the surface of the screen and the deep blue underwater backgrounds of the South Pacific, an effect even more pronounced in 3D.

Perhaps it’s not enough to warrant shelling out 3D dollars to go see a movie that’s long been one of the best-selling home videos. If you have kids, you already have this at home. But “Finding Nemo,” back in theaters nine years after its release, is a reminder that sometimes “instant” and “classic” can go together in a sentence describing a great movie — “Finding Nemo” is a great movie, one of the best animations for children ever made.

A timid single-dad clownfish (Albert Brooks) overprotects his mildly disabled (shrunken fin) only son (voiced by Alexander Gould) to the point where Nemo foolishly rebels and is promptly snatched and tossed into the tank at an Australian dentist’s office.

Dad flees the comfort of his reefside sea anemone home, and with the help of a seriously absent-minded blue tang named Dory (Ellen DeGeneres), sets out to find his son. And the kid, with the help of a tank full of mentors (Willem Dafoe, Brad Garrett, Allison Janney, Austin Pendleton), plots his escape to get back to dad.

It’s a simple story, perfectly executed. Especially when it comes to the voices.

Dory — all halting, self-interrupting comical kvetching, written specifically for DeGeneres and animated around her gestures — steals the movie. “I suffer from short-term memory loss. It runs in my family … At least I think it does … hmm. Where ARE they?”

It’s a grand quest filled with funny, broadly drawn but wise characters — sea turtles that speak “Surfer Dude,” Australian sharks trying to turn vegetarian (Barry Humphries, and see if you recognize Eric Bana), a helpful, plucky pelican (Geoffrey Rush).

Plus, what wonderful messages. No matter what, “just keep swimming.” “Trust, it’s what friends do.” And kids: “You can’t hold onto them forever.”

So don’t think of “Nemo” as just another 3D conversion. Think of this re-release as an encore, a handy touchstone for you and your kids. “Finding Nemo” was and remains the gold standard against which all other modern animated films are measured, a classic from the day it premiered.

“Nemo” is preceded by the new Pixar “Toy Story” short “Partysaurus Rex,” a hilarious dinosaur-out-of-water ’toon about the mild-mannered T-Rex toy (voiced by the ever-meek Mr. Wallace Shawn) who breaks out of his rut when he is picked to join the bath toys for a wild night of suds and excess.)