Distilled water is missing all of the the essential minerals and is not good for fish.
If your water is too hard you can add some peat moss in a filter bag to the filter or just hanging in the tank, but best in the filter. It is a natural way to soften water, much better than liquid chemicals that give you a short term solution, but are gone in a couple days.
If you age your water before doing water changes add some peat moss in a filter bag to the container you age the water in as well, that way it is already softened some before being added to the tank.
Replace the peat moss once a month.