RO water and Onyx sand


Prolific Poster
Apr 1, 2005
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.