Fishkeeper
Crowing
Feeder goldfish can cause a vitamin deficiency if a predator fish eats a lot of them. It might be vitamin B? Something that gives them their color inhibits the vitamin uptake, if I remember right, and I'm not sure if that would extend to vertebrates. Feeder fish are generally infected with all sorts of parasites, also, and dosed with medications that may not be safe for your ducks.
Also, I'm not big on feeding animals that are able to feel pain to other animals while still alive. Yes, I know they'd do that in the wild, but this isn't the wild. You can kill things first.
If you absolutely have to give them fish, I'd go with those frozen bags of tiny fish. You can get those out of the fish food freezer at most pet shops. Given the size of the ducklings, though, maybe get frozen krill or shrimp instead.
For a treat with no welfare concerns (due to more than likely not feeling pain), I'd suggest mealworms. Live or dead are both fine.
Also, I'm not big on feeding animals that are able to feel pain to other animals while still alive. Yes, I know they'd do that in the wild, but this isn't the wild. You can kill things first.
If you absolutely have to give them fish, I'd go with those frozen bags of tiny fish. You can get those out of the fish food freezer at most pet shops. Given the size of the ducklings, though, maybe get frozen krill or shrimp instead.
For a treat with no welfare concerns (due to more than likely not feeling pain), I'd suggest mealworms. Live or dead are both fine.