It's ok, you want to reconstitute it with minerals. Some use tap water to do this, others use calcium carbonate, baking soda etc. Kent marine has a product called RO right, I use some cheap junk I got from the lfs called Electro-right for GH, and Seachem alkeline buffer for KH, it never dissipates like baking soda did in my tank. I only bring mine up to 3dKH and about 7dGH.
Just read up on reconstituting RO water and you'll see all the options out there.
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