I don't know if this will be useful to you, but I'll tell you want I
do, as I use five-stage RO/DI water with absolutely no dissolved minerals in it to start.
The chemicals I use are ACS Calcium Carbonate and ACS Potassium Bicarbonate.
I try to keep the TDS as low as possible, and to mineralize my water, I like to avoid adding chloride or sulfate.
I also dose half the recommended amount of Seachem Fresh Trace
during the week which adds calcium and magnesium, along with Flourish and TMG.
I have found via testing that my magnesium levels are around the 15-20 ppm mark as CaCO3. It's been discovered that magnesium levels greater than 40 ppm as CaCO3 can cause complete stunting in some plant species. So I don't want or need to add a magnesium containing compound during water changes at all. To test for magnesium levels, I do a Lamotte GH test then a Lamotte Calcium test, then subtract the Calcium test results from the GH test results.
I change around 16 gallons out of 42 once per week. I add my CaCO3 and KHCO3, then do a GH and KH test, then adjust next week's levels by using percent calculations to achieve my target levels (GH 60 ppm as CaCO3 and KH 40 ppm as CaCO3).
To measure the CaCO3 and KHCO3, I use a gram scale.
I did try Potassium Carbonate (K2CO3) but it seemed to cause stunting. Plants show a very strong positive response to ACS Crystal KHCO3. My only guess as to why that is is that it probably has something to do with K2+ ions vs. K+ ions.
How to get CaCO3 to dissolve? It is insoluble in plain water, but in water with CO2, it forms Calcium Bicarbonate, which can only
form in liquid, and is
soluble. I stir it up really good in some RO water, then add it to the tank when CO2 injection is on. Water is initially very cloudy, and clears close to 100% within 1 hour and 100% within 2 hours.
Why do I not use Greg Watson CaCO3? It's not really CaCO3, but Dolomite, which has a significant amount of magnesium in it, more than what I want or believe is good.
Where to get ACS chemicals? Many lab suppliers refuse to sell to individuals (even though CaCO3 and KHCO3 aren't even regulated) because if they allowed that they believe their customers would make bombs and drugs. Whatever. There are some who will, such as chemsavers.com, clarksonlab.com, and sciencelab.com (who I don't recommend).
That's my own personal methodology so take it as you will, as some people say, my advice is worth exactly how much you paid for it. It works great for me and I am personally happy with it. I have experimented with different chemicals and levels and I believe I currently have it optimized. I can finally grow Rotala macrandra
like the invasive weed it's supposed to be, as opposed to the "very difficult, experts only" aquarium plant classification.