It looks like he's already adding tap to his RO, that's why the KH=10. Since your KH=10, I'd guess that your GH is somewhere in that vicinity. Just go get a cheap GH test kit, and don't bother with separate Ca and Mg kits. My Florida tap water is kh=7 and gh=10-11. So, don't be took afraid of a KH=10. Most plants will do just fine in water like that.