There isn’t a simple evolutionary definition of “fish”, not the same way there is for, say, mammals. If you found the common ancestor of everything we call a mammal and said “everything descended from this one is also a mammal”, you’d be correct. If you did that for everything we call fish, every animal in the world would be a fish. Also, we decided which animals were fish mostly on vibes, so without a clear definition you can pedantically argue that everything is a fish including mammals.
There isn’t a simple evolutionary definition of “fish”, not the same way there is for, say, mammals. If you found the common ancestor of everything we call a mammal and said “everything descended from this one is also a mammal”, you’d be correct. If you did that for everything we call fish, every animal in the world would be a fish. Also, we decided which animals were fish mostly on vibes, so without a clear definition you can pedantically argue that everything is a fish including mammals.