Nothing that would be safe for your ducks. Your best bet is to just keep changing out the water on a daily basis and refilling it with a hose. I wouldn't even use rain water, since there have been instances of protozoa and other harmful bacteria found in it.
Wow, really? I wonder if that's true for call ducks, too...I mean, ducklings are pretty peep-tastic regardless, and call ducks even more so! But Wobbles practically sings nonstop, especially when she eats. It's so cute. <3
What the duck are you talking about? Bird jokes are so corny, it's un-bill-ieveable. I don't mean to ruffle your feathers, I'm just sayin' it'll get you goose-egg.
(Oh god stop me before I pun again)