These days, properly balanced feeds are readily available from a feed supply store. Be sure, especially with young fowl, that it is actually for ducks, not for laying chickens. Store-bought bread of the spongy sort is very dangerous for all types of fowl, especially ducks who like to get their food wet before they eat it. The readily bought sort of bread acts like a sponge, getting bigger and posing a serious threat of choking. Home made bread is much safer.
Ducks are good foragers, finding slugs and worms and such during their rambles.
They are probably also prone to obesity as they seem to always want more grain than is good for them. My mother once thought it would be a good laugh to throw the ducks into my bath with me when I was about 7 or 8. It was funny, but I had to get out immediately as the water was instantly clouded. The water must be at least deep enough for a duck to get its entire bill under the water in order to clean out its nostrils. The water should be deep enough for a duck to immerse itself and ideally would also have some room for swimming.