I use RO water, so 0 degrees of KH. I also use an active substrate that also affects my KH; although, in what way, I have no idea.
Years ago, I injected CO2 and cared little for the fauna so I adopted an Estimative Index (EI) style of keeping my plants: I assume my tap water has nothing and proceeded to add KH, GH and nutrients as I observe plant health. Everything was spectacular! Now that I house and develop an affinity for finicky shrimps/fish, I have to find some kind of compromise between the fauna and flora. I can't just EI my RO water and have the annoying fauna happy. But if I give them the 0 KH they so love, I fear that I may gas the stupid little things to death.
So my dilemma: I rather not add any carbonate if KH is no longer important: fauna likes it and I have one less thing to add. But if I have to add KH, then what is that pesky active substrate going to do to all that lovely CO3?