as far as hardness is concerned, there's two types: GH and KH. GH is a measure of the calcium and magnesium in your water and KH is a measure of the carbonate level. unfortunately, while the test kit you're using measures only the presence of Ca and Mg, there may be many more cationic species in your water than you're able to test for. Sodium, Potassium, Titanium, Nickel, Aluminium, Manganese, Copper, Tin are a few you may have heard of. the sum total of all of them would be your total dissolved solids (TDS) and it is THIS function that has the most significant impact on fish ... much more so than Ca and Mg alone. these cations are attached to anion species such as carbonate but may also be silicate, phosphate, sulphate, chloride and others. again, while you're measuring carbonate as KH there are other disolved inorganic compounds in your water that you're not testing for.
when you run water through a home water softener, the anions Ca++ and Mg++ are exchanged for sodium (Na+) but notice the difference in electron charge ... Calcium is a divalent anion (++) but sodium is monovalent (+). this means that when the calcium is exchanged, TWO sodiums must replace ONE calcium or magnesium. the net effect is that while you are softening your water with respect to your laundry and those white spots on your faucet, you are actually INCREASING the total dissolved solids of your water.
fish don't do laundry and thus they are more affected by the TDS of your water than they are with which particular anion it contains.
all that said, tens of thousands of hobbyists do in fact use water from their water softener without any problem at all. the amount of salt isn't very high and the vast majority of fish will not be negatively affected by it.
