I used to put sodium salt tablets in a plastic cup in place it in the tank in a futile effort to soften water as recommended by an lfs (they did it in all their tanks)
I eventually tired of having fish get 'trapped' in the cup or spilling it's salt all over the substrate, plus it didn't really seem to be making much difference
ap's pillow is probably just sodium or potassium salt tablets inside a tight nylon mesh bag ... it probably works better than the cup 'o salt method, but still, you're putting salt into your aquarium, your fish and plants might not like it
try softening your water with peatmoss and/or driftwood ... althougth instead of putting salt into your water, these put tanic acid (tanins), which will yellow the water and reduce it's ph, but you can counter the yellowing with filter carbon or water changes