I am using ADA's Aqua Soil in my 30G. The soil used to keep my GH, and
KH down to barely detectable level, and I regularly dose CaCO3 to keep
everything happy. Now the substrate is aged, it seems that the soil is
slowly losing ability to keep the KH down, and now its back up to 3
which is about the same as my tap water. However, GH is still way low.
So my question is what should I use for supplementing Ca without
affect KH? I also dose MgSO4 regularly, but I guess using Mg alone to
make up the GH won't be a good thing, right?

Thanks a million in advance

Cheers
Kenneth