reverse osmosis is removing calcium carbonate ? forum search is broken so I can't see the earlier discussion on them, but Amazon reviews says
They remove everything the re mineralise to add in a little calcium and magnesium as otherwise the water is too pure and isn't great for drinking / espresso. Not sure theres anyone/many people on here but allot on uk coffee forum, in-fact the keep doing group buys there.
Personally i'm using RO water from a local fishkeeper with some Sodium bicarbonate and Magnesium Sulphate added back in.