Hydrogen sulfide perhaps?
I Don't Have Enough experience to know for sure. But with what I've read that usually comes with a smell, which I have absolutely no smell or ill effects so far. Just the black spots as of right now, I vac them and their gone for a few days. I do vacs and 30% water changes twice a week and all my parameters seem to be fine ( I use api master kit) but do not check for kh or gh so I'm not sure about that. But I am still REALLY green to all this.