Everytime you change 50% of the water in your tank you are adding 20 ppm of nitrates. If you do that twice a week, with relatively high light, you likely have plenty of nitrates. But, if you only top off the tank as water evaporates, the nitrates will be used up within a week or so, so you will need to dose nitrates.
With water that hard you sure don't want to just top of the tank with tap water, or you will soon be able to walk on the water! The calcium will keep on building up.
None of that data says anything about potassium (K), so I think you can assume that you need to dose potassium, as K2SO4, for example.