You noticed that too huh?
IMO without using a TDS meter you have no idea what your parameters are.
You would need to test the tap water first with the TDS meter, Then you would add xxx distilled water, test using TDS meter if the water is still too hard add more distilled if too soft add tap water, and you would need to do this every time, and if you dont your water parameters will fluctuate and that will have adverse affects on your fish.
Changing water chemistry is not as simple as adding xxx cups of this to yyy cups of that, and anybody who tells you different has no idea what they are talking about.