RO water and Onyx sand

I've been using RO water as of late when doing water changes. The past two times I've checked the KH in the tank, it's been at 6 after roughly 35g water changes (of a 75g tank). I've only used RO water so far. My main concern is if I'm not using tap to cut the RO water, am I going to run into Ca or Mg issues using Onyx sand substrate? I assume it's buffering the water, hence the reason the KH won't go any lower. I'm trying to get a feel for the buffering capacity and if there will be a need for additional ferts before the plants shrivel up on me.