Purina makes both a medicated and non-medicated flock raiser that can be used for ducklings from the first weeks. I think they recommended two or three weeks only of the start n grow.
I see some people feeding game bird food, is that better?
I was thinking of using chick starter partly because I may have chicks here soon too.
I got the idea that ducks can just get too much of the medicine because they eat larger amounts and the dosage is calibrated for a chicken.