
It might be lime or calcium. The German word is Kalk, but it’s basically the hard water, as you say. Yes, we were doing exactly that (the water is very hard here), and tried to remove the aerator in order to clean that too, which we hadn’t done before, and that’s how it all broke.
I’ve now made progress in breaking off another piece, but we’re getting closer to just calling a plumber.










I did that and broke off another piece, but I’m having trouble getting that piece out now. We’re getting ever closer to calling a plumber :).