That's a tough one...I feel for you! I think the fish would train, but the ducks are very proficient at gobbling up food. I'd bet your feed bill could easily double (or triple) with ducks at the table.

Around here, the neighbors would be notified of my intentions with hopes that they would pen the ducks up during feeding time, but I know that can be difficult to agree upon and sustain.

I can imagine a floating cage under the feeder, like a feeding ring with a chicken wire basket over it...feed goes through the chicken wire, floating ring contains the feed for the fish, and the wire keeps the ducks from getting to the feed. Maybe your neighbor has some materials to donate and a hand at building it. Otherwise a duck relocation plan would be put into place.

Fish on!,
Noel