OK let's break it down. Are you feeding ducks and drakes or just ducks? If you have drakes, this has too much calcium. Drakes or ducks that aren't in lay require only about 1% calcium and too much could cause Kidney failure. Also, what age of ducks are you feeding this too? Anyways, too much protein can cause leg problems. If I were you, I would stick with a feed with about 16-17% protein. But that's just me.
Before I changed feed to a Non GMO I used Purina Flock Raiser for years. Never had any problems with it I am pretty sure other use it too @Ravynscroft ? The reason I liked it was it is formulated for all poultry which is nice when you have a mixed flock.
That is an absolutely fine feed for ducks! And @Miss Lydia is correct, I use Purina Flock Raiser 20% protein for all ages, no problem... and drakes even eat layer feed, without issues as well...
Problems with too much protein is when it is in the higher range of 20's, 20% is actually my optimal, especially for young ducks and molting ducks, I wouldn't feed lower %...
Leg problems are from not enough Niacin, since most feeds formulated for chickens doesn't have enough for ducks... but the flock raiser contains higher amounts, I have only seen issues when the ducks themselves as individuals have a need for extra higher Niacin... but at several months old, those ducks should be just fine...