Hard water can't be made into soft water without using RO/DI water for water changes, or mixing RO/DI water with tap water to get the hardness you want. Water hardness is due mostly to calcium and magnesium in the water (GH), or carbonates in the water (KH). You have to remove some of those ions to soften the water appreciably.
Some people use peat to improve hard water, but that also gives yellowish colored water. If that is ok, you could put peat or peat granules in your filter and improve the hardness somewhat.
Now, why do you feel a need to soften the water?