I filter through peat and it lowers the Kh very well IMO. Peat Pellets are easiest to work with, however can be quite pricey.
The best prices I've found is buying the Laugana Pond peat.
Pond Chemical Filtration Media & Water Softening: Laguna Peat Granules
It's 4.4 lbs for $15 vs the Fluval Keta or tetra boxes that are 10-20$ for 14-25oz containers.
Put it in a Filter saver bag or nylon stocking and put in your filter where you would put the activated carbon. I even open the Whisper filter bags and add some to those in my nano tanks.
Good luck
Last minute note: it will stain your water brown