Can you prove that you're not a fish?
It goes as follows.
To prove that something has a certain property, there must be a metaphysical point of reference by which we can prove that that thing has a certain property. For example, if you and I were in a room together, I could prove that you were wearing a shirt (hopefully) by referring to the shirt on your body. You could prove that I had brown hair by referencing my brown hair.
But how do we prove that something does not have a certain property? What metaphysical point of reference can be used to prove the absence of a property? In the debate question, what can we plausibly use to prove that we're not fish? We could send the OP a picture of ourselves. But how does the OP know that we're truly fish, and not just hiding our fish forms in the veil of human disguise? It sounds ludicrous, and indeed it is ludicrous to believe that we are fish, yet we cannot prove we are not. What this means is that everything we take as truth in our life is not actually proven, and our whole life could be a lie.
This is a problem that is addressed in the idea of negative objects. The metaphysical point of reference that proves X is true is positive. The metaphysical point of reference that proves X is not true is negative. This means that for every positive fish (or regular fish), there is a negative fish to compensate its existence. Indeed for every object, the theory states that it has a hypothetical negative counterpart to compensate its existence.
The main contention with this idea is that how can an object be used to prove nothingness? Isn't that contradictory? I disagree with this contention on a mathematical level. Think of objects as a number line, they can be positive or negative. Even in physics, this is true, where we have positive and negative energy. So instead of suggesting that a negative object is nothing, I would suggest that said negative object is truly and mathematically negative, allowing it to exist.
Of course I disagree with the whole negative objects theory anyway, seeing as I don't feel it necessary to prove negatives, but it is an interesting topic. I don't know the name of the reasoning, though I suppose it would be a form of anti-realism. Ludwig Wittgenstein did some work on this, so that may be worth a look if you want to know more.