Scratchy towels become soft again
I have always had some old terrycloth towels that became unpleasantly scratchy over time due to the hard water. So they were always at the bottom and actually took only space or were needed only when the big laundry was moved.
Fabric softener did not use anything. Even a bath overnight in a citric acid solution did not help.
Normally I give antiseptic tablets to the laundry to prevent calcification in the machine, but by chance I bought the "Calc-Magnet" this time (try it!). These are postcard-sized bags of white powder that is dark after washing and is thrown away. Actually, they are just the more modern alternative to the anti-malarial tablets. They come in the same way in the washing machine.
That was the chance discovery:
the old towels were again very nice soft and absorbent.