I’ve tried different variety of oyster shells. Some are used for gardening, so that wouldn’t be good for ducks.I had been providing oyster shell calcium when my other duck was still laying. I don't ever recall seeing the duck that laid a soft shell egg partake of it.
I'm a bit freaked, as my first batch of ducks laid eggs with strong, thick shells, and always in the morning.
I'm also wondering how efficacious the oyster shells are. My other duck that just stopped laying last week, was laying eggs with painfully thin shells and she was the one I always saw eating oyster shell.
I think the crushed shells have to be pretty fine in size. This is the brand my ducks seem to really love:
https://www.amazon.com/gp/product/B00UIM7N0A/?tag=backy-20