I have a rubber tub that the ducks can swim in, and clean themselves, But they're not even going in it, just drinking. I think they're use to bathing and swimming in a tub, from the previous owner. I don't want them to have to come in to take a bath, they're outside, they should swim and bathe in their own space. Their 2 ish months old.
Any advice?
Any advice?

They may swim. Just not when you're watching, because that would be too kind of them. 

the only time they get clean is when it rains
other then that all they do is clean just their heads.
