The most reliable way to reduce hardness is to use RO water, or a mix of RO water with your tap water.
RO can be expensive and inconvenient, whether you invest in a home unit or buy regularly from an LFS.
Unless you have your heart set on a particular soft-water fish I'd recommend that you simply look for fish that will thrive in your tap water. With a little care and research you can still keep lots of different species.