What argument? Over the definition of “fish”? There is no one such definition. It can be pedagogically useful to refer to any gnathostome as a fish, and sometimes I do that. It’s hardly a strange definition, unless of course you’re a creationist. Do you think humans are apes? How about monkeys?
Incidentally, I notice that you ignored most of my reply in order to concentrate on the most trivial point. Is that wise?