I feed my ducks Purina Sunfresh recipe...It's in a green paper bag and you can get it at any Farm Supply Store. It is for 'ALL Backyard Chickens, Ducks, Turkeys. It is a crumble form. I also add Scratch Grains to the food.
about one cup of SG to about a two Gallon jug. I dont ration their food and fill their dish about 1/3 of the way. They eat what they want and when their full, they go nap or swim.
They love it and are very healthy. I have a run for all of them wich has plenty of dirt so they don't get Oyster shell. The egg-shell's are normal in hardness.
For a treat I give the ducks chopped lettuce with some frozen corn and peas mixed in with it. The Ducks go nuts and will scream at me if I dont give them their morning veg's..........What can I say, I spoil them rotten.
I tried the pellet form and they weren't too keen on it but ate it although it's less waste. the sunfresh recipe is 16% nutrients. I had two kaki campbells and loved them.