I don't see how KH can ever be below zero. Intuitively I think the KH could only approach zero asymptotically, never quite reaching zero.
No I don't mean less than 0. I mean 1< x >0. Less than 1 greater than 0 degrees of carbonate hardness. My test kit is accurate up to 1 degree. But I can't test in PPM which I suspect will give me a more accurate amount. Albeit a very low amount.