Egg eating ducks

Has ducks eating their eggs ever been a problem for anyone. Am I going to have to cull them all? There are 19 of them so have no idea which one/ones is the culprit.
I have increased their grit by loads, I am mixing it in their feed and have pots all over the place. Do you think this will harden the shells enough to stop them?

I've never had an egg-eating duck. How d'you know it's a duck that's eating them?

I caught one doing it when i let them out. I wondered why I never had many eggs. The wild birds apart from the tiny ones can't get in at night so its not the crows.