I don't believe that anyone really has 0 KH because the pH of water with 0 KH would be about 4.3. I suspect that your KH is below the test limit of the test you are using. Anyway, I have no experience using such low KH water. I mix my RO with tap to get 4 deg KH.
now the real question is what if the KH was 0, how will the effect the co2 and plants. i know my test kit cannot messure lower than 6.0 PH, but API test kit for KH always change color with 1 drop. am sure KH levels are almost 0, because am using 100% RO.