Replacing one feeding with fish should be fine and good for them.
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I was thinking about getting my ducks a treat of live fish. Do they eat them? And what kind?
You should be aware that feeder goldfish at the petstore are kept in dirty, over-crowded conditions. They could not survive like this naturally so they are medicated with copper. Fish can deal with copper in their environment but not all other animals can. Reptiles with a diet high in feeder fish have died of copper toxicity. I do not know if this has ever happened to a duck before, but I would advise against giving them many "feeder fish"- they are meant to feed larger, predatory fish, not other pets.
I do not know how minnows are raised, so I can't say anything about them.
How old do ducklings have to be before I can feed feeders to them and do I put the water in the fish bag in the water they swim in or strain the fish
As awesome as it is that you care so much for our birdies' welfare, I don't think it's a big risk for ducks. For one, not all farmed goldfish are treated with copper. Yes, they are raised in cramped conditions, but as hardy bottom feeders they can tolerate some pretty icky conditions that most other fish wouldn't.
Second, for the ones that are treated with chelated copper, the amount of copper that any one goldfish would contain in its tissues is completely harmless. It's true that copper toxicity is a risk in reptiles, as they are cold-blooded and thus cannot remove the toxins from their body as quickly or efficiently as birds or mammals would, and you only see it in animals whose owners feed them nothing BUT fish. That's why young snakes and pacman frogs are recommended crickets and pinkie mice instead of goldfish.
Third, ducks eat things far more disgusting and dangerous on a daily basis. Things like parasite-laden snails, mosquitoes, and their own poop. I think a few feeder fish every once in a while makes a great treat!