We are going to use a 40 gallon stock tank as a 'swimmin hole' for the ducks (3 or 4 Swedish). I found this product on the same site that sells the stock tanks. In the description it says it's "completely safe for horses, cattle, and other livestock, as well as humans, pets, birds, and fish", so I'm guessing it would be ok for ducks. Has anyone used it?
http://www.enasco.com/product/C25950N?ref=recent
http://www.enasco.com/product/C25950N?ref=recent