RO water contains no KH or GH. You need to add products back to your RO water to raise KH, GH, and preferably trace elements.
Many people use baking soda to raise KH. I like to use Seachem Alkaline Buffer, which is essentially baking soda, but it's crystalized so it doesn't cloud water, and it's a bit more refined than food-grade baking soda. People often flame me for using it, but I like it and it's my preference.
For GH, there are products you can use like Seachem Equilibrium and Kent RO Right. These contain large amounts of potassium, which is fine for some people. I like to dose potassium seperately, so I use Kent Botanica GH+, which contains only calcium and magnesium (the main salts that show up on GH tests).
For trace elements, I use Seachem Fresh Trace.