Hard to say - Romy makes those sometimes and she seems to be okay. I have brought her to the vet about the soft shell thing - brought one of those in for them to see. We try to keep her calcium levels boosted, because it is thought that low calcium puts ducks at higher risk of egg binding, prolapse, and bone and heart problems.
I use 23% calcium gluconate liquid, 1 teaspoon per half cup of water, mixed into the feed. And there is a prescription - Neocalglucon - you can give also. Romy likes hers in half-mashed peas.
Something I am trying, instinct more than anything, is ground flax seed in their food. No soft eggs since I started that, but then again, Romy went broody and stopped laying . . . so no double-blind experiment to go by. Vitamins, trying to keep them healthy is pretty much what I am left with. I think sprouted feed might be something good to try, also, and getting back to fermented feed, too.
I feel that in several years that duck veterinary medicine may progress beyond what it is, just as domestic cat veterinary medicine progressed. I would like to think so, anyway. And egg issues are an area where it would be lovely to have better diagnostics and treatment. In the meantime, sometimes I just go with my gut in addition to the vet's advice.
Thanks again. I can easily do ground flax seed. They love the koi pond and last year I had no problem with soft eggs, but last year we had a lot more stringy algae in the ponds. They do love that and the snails. again hardly any snails this year. Have no idea why the difference, but that would change the makeup of the vitamins they get.