I get the impression from the other comments that a popular YouTuber made a recent video about it, where I'm guessing he tried to be a smartass about some of the implications of "fish" being ambiguously defined, and a bunch of his more credulous viewers have got the wrong end of the stick and ran with it?

I mainly know the idea from an old QI episode that explained it as "there's no such thing, biologically speaking", which in turn became the title of the show's researchers' long-running podcast "No Such Thing As A Fish".

Well, yea that. I know Hank Green likes to bring it up and he’s popular, and it’s come up with a bunch of other content creators, too. It’s fun, technically true, and enough people don’t know it yet so it’s great to bring up in the real world.

Plus it’s super cool to explain how our eyes are built for the ocean and they had to get a special lens on top to fix it. Or how we carry the sea with us in the form of all this salty water. Or the whole swimbladder/lungs/guts thing relationship I mean they’re all quite fun, really.

I also like explaining that octopuses are molluscs and, because we say that molluscs have one “foot”, it’d be more appropriate to say that octopuses are really just going around the world with eight funky toes and not eight arms.