のか vs か in asking a question

Question

What's the difference between using のか and か in asking a question. From what I know, の and か really turns the sentence into a question, the latter being the polite version. So what's the point really in using both の and か

それは魚じゃないのか それは魚じゃないか

Answer 1

Simplified answer for now: 「それは魚じゃないのか」 feels to me like 'Wait, that's not a fish?' or 'So it wasn't a fish after all.' 「それは魚じゃないか」feels to me more like 'That's obviously a fish!' (maybe someone said they'd bring chicken for dinner and brought fish instead).

EDIT: Okay, asking a question with のか often indicates that something is contrary to what you originally thought. Maybe you thought dolphins were fish, and somebody tells you they're mammals - you might say 「魚じゃないのか？」 then. Or your spouse told you yesterday you were having fish tomorrow, but today they say they're going to get beef - that's another situation in which you might say it.

Meanwhile, じゃないか, a lot of the time, isn't so much a question as a strong assertion. Somebody asks you if sharks are mammals - 「いや、魚じゃないか！」And, of course, relating to my previous examples, your spouse told you before they went shopping that they'd get some chicken, and they come back with fish instead - 「魚じゃないか」. I feel like this also inidcates a bit of surprise, but maybe I'm wrong on that.