I've never done it myself, so take this for what it's worth.There are numerous products (such as Dennerle GH+, Seachem Equilibrium, Salty Shrimp) to remineralze water and I've used the three I named. All add magnesium and calcium without adding carbonates like you are planning to do (actually, I think the Seachem product does raise KH a bit). No reason I can think of not to "roll your own." I ignore package instructions and just add until I'm between 130 and 140 on a TDS meter. Unless your water is very hard Cardina shrimp are about the only need for an RO/DI system for freshwater that I can think of. I have soft neutral tap water and still have to use it for my crystal reds. It works better for the shrimp than the plants; the same plants grow at much better rates in my tap tanks than remineralized RO tanks with identical lighting and care. I use the RO wastewater -which is all the TDS that didn't make it through the membrane -on plant and snail only tanks and they seem to love it. All that to say, most plants do their thing more slowly in soft, acid water by most accounts but if you want CRS shrimp it's a necessary evil. Especially with tap water with values such as you have.