Everyone's experience is different, but if you're having leg problems niacin is an excellent first step in treatment--even if it doesn't solve the problem, it won't hurt, and higher niacin in general supports good leg health in ducklings (but not too high--you can overdose--details below). I do not mean to criticize those who do fine without niacin, but I will say that I have also raised hundreds of ducklings and on the one occasion when I decided not to bother with the extra niacin, the birds developed leg problems which were quickly corrected by adding back the correct amount of niacin. And it is not just me. The most-respected waterfowl expert in the nation, Dave Holderread, also recommends higher niacin for ducklings.
You have several options for accomplishing this:
1. Mix the chick feed 50/50 with game bird starter. The game bird starter contains high levels of niacin, but also levels of protein that are too high. By mixing the two, you balance it just about right for waterfowl. This is what I do because it is simple and effective.
2. Add brewer's yeast to the feed. This is also simple, IF you can find brewer's yeast (try health food stores or beer making supply if your town has one). Just sprinkle it on top of their food. You can't overdose on brewer's yeast.
3. Add niacin directly to their water. Buy niacin at the pharmacy, try to get the NON-flush-free, and also NON-time-release (this gets harder and harder to find). You need it in gel caps because you're going to break the gel cap into the water and dissolve the contents in the water. Don't worry if it doesn't all dissolve--I've found it's quite effective even if there are still bits of it settling on the bottom. The dosage is about 100 mg per gallon of water. It doesn't have to be precise, but don't overdo it.
4. Feed them Purina Flock Raiser. I have had mixed results with this among people I've talked to, but it is recommended by Dave Holderread and he is rarely wrong, so it's definitely a potentially viable option. If you end up with niacin deficiency anyway, you can always add in extra by one of the other means.
Niacin deficiency, when caught early, is easily corrected by adding the correct amount of niacin. Of course, if you're raising lots of birds, it's way easier just to prevent it to begin with, plus birds thrive best when provided the proper nutrients, even if a deficiency isn't clearly visible. If niacin deficiency symptoms are allowed to persist without correction, they can develop into permanent debilities.
Good luck with your babies. It's quite possible her ankle problem is NOT caused by niacin deficiency (in fact, niacin deficiency usually causes a generalized weakness and shakiness in the legs rather than location-specific distress), but the proper quantities of niacin will help develop leg strength which may aid in her healing.
I hope she does great for you.