any form of a CA Mg Carbonate. Marble is ok, I use Dolomite because its not as soluble as calcite or limestone or marble. It lasts for a good 6 months on a 100 gal tank. (We only have 2 people in the houe full-time and occasional visitations from the younger.)
We run a fairly large stock farm and keep the water to the fields on a water level seris of faucet controllers. (We will use about 10 gal a day pwr each sheep n maybe 25 gpd per steer. We MUST have water that is copper free-because copper causes anemia in the sheep (not the cattle though).
Our well can produce about 40 gpm with about 10 ft of drawdown (Its a great yield). We are, hoqever, in a metamorphic rock belt made up of micaceous layers so its fairly acidic naturally (Itll buffer itself on anything you have that is metal, so if its a new house I hope you put in PVC pipes for water, in the house)
You should find out your wells yield because if its low you probably have metals in solution s your hardness and TDS.
Our pH was 5.5 with copper and with a 100 lb CaMgCO3 "kibble size" charge AND a GAC filter and a tubular final" final polish" filter , we have great water.