The only thing I can think of is to add peat as a water conditioner, sometimes driftwood as long as it is not from an ocean. As mentioned previously you can use RO water. I have found using the soil substrates like Fluvial Substrate or Tropica Aquarium Soil (not necessarily recommending these) does seem to reduce hardness as well as pH. It always seems that adding material to the water is easier than removing it, softening the water has always been more difficult, you need to add something that will capture or bind with the dissolved solids, in the natural world that is typically organic material in the water. You could do what I did once and that was to change my focus to African Cichlids. If you are stuck on soft water loving fish then RO seems the best bet.
