How much Brewer's Yeast for ducklings?

[tmb3fan]

I received my 20 ducklings and 4 guinea keets yesterday. They are eating non-medicated chick starter. I have been crushing Brewer's yeast tablets and putting in the food, but I don't know how many tablets to how much food. For those of you who have used the tablets, can you answer this question?

Thanks,
Teri