What's the opposite of a plankton? Ans: a nekton . A whale is a nekton, that is a free swimmer, and in this sense it is a fish. It's just a matter of usage. Just a word. Similarly, among the plankton, which by definition are drifters, one finds many classes of organisms, including crustacean larvae.
I’ve always been amused at the way people who grew up attending modernist schools think historical people are “wrong” for calling a whale a fish. Whales are not Pisces, but “fish” was a term used for any sea creatures, historically.
The ancients and less-ancients were quite well aware that there were major biological differences between whales and Pisces; the name “dolphin” means “womb,” in Greek, for what the Greeks called them, translated into English, was “womb-fishes.” Also not Pisces are shellfish, crayfish, starfish, jellyfish, etc. Some eels are Pisces, some are not. And “Pisces,” as a phylogenic class has been discarded, anyway.
If sometime around the 1950s, elementary school teachers decided that they would only use “fish” to describe Pisces, that’s fine. But throughout the whole of English history up until that time, “fish” meant a category of creatures which included whales, polyphylitic as it was.
Will they someday call us wrong for calling both earthworms and pinworms “worms” even though they are not related?