We have, or rather had, a nice pond in the garden. It's still a pond but it's not really very nice. You can't really see from the picture but it's the colour of pea soup. I'm afraid I put this down to the ducks, who were allowed to spend quite a lot of last year on the pond, and their somewhat careless personal habits mean that the water has been "enriched" and is now too high in nitrogen which has caused the algae to flourish, hence the green water.
I've had ducks on the pond before and not had this ill effect, but I think really you do need to have a source of running water such as a stream to make ducks really at home without running the risk of this kind of problem. At the moment the ducks are in a run and have their own childrens paddling pond, which I can empty and refill at regular intervals. I'd prefer to let them free range, but I know they'd be straight on the pond, if they got the chance so it's not an option, for a while anyway.
So if you have ducks, and you don't want to end up with this