He's not bringing something else to compare. You can rephrase the discussion like this:

Claim: We don't need to eat fish. It is not necessary for humans.

Counter claim: we need to eat fish because humans need nutrients such as omega 3 fatty acids.

This is a direct dispute. The claim and counter claims have not been changed. They are both directly on topic.

Here is an example of whataboutism.

Person1: Biden says 1 + 2 = 4! Biden is wrong! Person2: But Trump said 1 + 2 = 1000000! He's even more wrong!

This argument does not address the claim that Biden is right or wrong. Not does he talk about the problem. Person2 is misdirecting by bringing a separate person as form of attack. They're both wrong. Trump being more wrong does not validate Biden's incorrect answer. Like I said, whataboutism is a subtype of ad hominem attack.

It's also possible person2 could've said: What about Trump? He said, 1 + 2 = 1000000!

It's easy to formulate whataboutism by using the words "what about", and it is done so commonly. That's why it is called whataboutism. But again, what is being said is important, not how it is said.

A person3 could say: What about 3?

This is not whataboutism. He's showing what is his side to the argument. Even if the person3 gave the wrong answer like "what about 2?" It is still not whataboutism as they are still talking about the problem rather than misdirecting.