Pootang is basically right.
so is Harryfish.
If you have an ATO, lid your reservoir tightly, and put up to 2 tsp of Mrs. Wages Pickling lime in your ro/di water.
You MUST set your tank levels by hand-dosing alk and cal and mg. but once set, the kalk will hold it steady without further dosing. Test once weekly, and if the levels fall, test mg. Raise mg, then raise alk and cal by hand-dosing, and let the kalk drip continue.