Water softener salt just exchange's the sodium ion for the calcium ion.
This is good for softening water to help make detergents suds up better, but doesn't really provide the mineral free water that is the same as the water in Amazon basin.
Oak leaves, or peat moss will provide tannins that are helpful (antibacterial), but do not really soften water, os lower pH significantly.
Mixing tap water with RO, or rain water can sometimes do the job.
Many aquarists that want to keep, and (especially breed) these Amazonian species wili set up RO, and/or elaborate rain catchment systems.
I collected rain water for soft water species I kept.

I must admit this is difficult when getting enough water for the kind of water changes large cichlids need, and I also soaked leaf littler to get tannins..

So because of my limited supply of soft water, the only soft water species I kept were small species, such as killifish.

So once I realized the limitations of my local water conditions, I tailored the species I kept, to match the water I was provided.
I have always "wanted" keep Uaru fernandenyepizi, but because of my tap water situation, it was not reasonable, so I always passed.
I know this sounds drastic, but the large percentage of successfully keeping fish, is water management, and providing correct conditions for their health.