Les, I really don't understand soft water/hard water as it has to do with minerals, it does apply though if soft water is mineral free; next problem is sourcing or converting. Food for thought.

M.Magis, wow, my 'net searches just turned up thousands of ways to get harder shells. When I searched your quote it opens up a whole barrel of stuff that indicates boron (or 20 Mule Team) may need adding to my diet as well. Quick searches point to cure for arthritis among other things.
Allegations that boron has been suppressed in Europe and banned in Australia in an effort to protect drug manufacturers from sales loss if public used boron in diet. Going to require much more reading.

What Magnis said is not only interesting, it's something to look into and research. BUT, you have said you have raised many chickens before without having this problem, with the same water source. Myself, I would give it some time and if the issue still persist, then do the detective work.Until then, whack them eggs hard lol.

I would try giving the chickens apple cider vinegar. We give our commercial turkeys apple cider vinegar. It might help soften the water a bit.

I read another interesting theory on this yesterday late (I fergit what site, there's a slug of stuff out there) but one of the few things I found that related suggested "rooster stress" and that could apply, I just got rid of six or seven roosters (can't say the other word here ) last Sunday and the ladies seem a lot calmer now.
The boys were mostly accidental, I bought feed store "straight run" pullets last April or May and not surprising some growed spurs. Had one male already and a daughter brought me another (rescued from being a "pet") so with only about 35 hens I had surplus bad-words and when I started filling cages I just kept the Bantam with the trout flies on his back.
Stress is one factor I had not considered.
I'll let it ride a week or two, cut back on the commercial feed and see what happens. Maybe give them some vinegar, that sure won't hurt them any, and apples are a source of Boron. That Boron cures arthritis and obesity in people so I ought to get some just for me.