If you live in the usa i believe there are brands of complete duck and duckling food available? Living in NZ where there are not, after a few years rescuing hundreds maybe thousands of ducklings I have found canned cat food to be the best, most bullet proof feed for ducklings.
Its top ingredient here is meat, which means it has the niacin they need for their legs. But it has an overall protein of around 9% so there are no issues with angel wing or ducklings growing too fast like you get with the ‘gamebird crumble’ - that is the only feed marketed for ducks here - but is actually super high protein and clearly marketed towards the fast growth of meat birds (with no consideration for their quality of life or health), with no more niacin than chicken feeds, qnd the mqin diffeence between the gamebird crumble and chick starter is just there is no coccidiostat medication (most of which are fine for ducklings but in much lower proportions than are in chick starter.
brewers yeast i have found to be hit and miss in terms of preventing niacin deficiency leg problems, it seems some works some doesnt. Have raised hundreds of ducklings on canned food and never had a single case of niacin leg issues or angel wing.
I just get the cheap brands here, in meaty non fishy flavours like beef, lamb, turkey, chicken (yes, birds. Friends with chickens tell me their chickens actually prefer the chicken flavour! though i have to qdmit there was no canned food here ith duck in it till recently, there is one now and i wont buy it or feed it to my ducks, chicken is ok to feed the ducklings, but duck is my line in the sand lol)
if you look at the feed it should list the ingredients and they are listed from the highest oercentage to leas, so here when i look qt canned food the ingredients list always has meat and meat byproducts first, then maybe some grains, but the meat is tops. It costs more to feed canned food (here anyway, like a clutch of 12 2 week old ducklings will eat 3-4 cans a day at $3-$4 a can per day) so sometimes i will mix up to 50% pullet grow (15% protein) with the canned food. If a bird shows signs of niacin leg issues i increase the canned food and the leg issues disappear. if any more than 50% of the feed is chicken (grower) pellets then i add in multi b vitamins. I dont use game bird crumble adter the first 2-3 weeks except as a very small overall percentage, because the protein here for that type of feed is 20-22% and too high for ducklings over 2-3 weeks of age. I dont feed layer pellets to ducklings (except for a day in a pinch ie if i run out or get wn unexpected out of season rescue duckling, till i can get to the supermarket, or the closest feed store which is a 40 min drive, for proper feed) because they contain far too much calcium for ducklings (but ironically for some laying ducks, not enough, and so i give my laying ducks additional grit as well as layer feed.
anyway if you have a small number of ducklings or canned food is cheaper than it is here, and the top ingredient in the list is “meat or meat byproducts “ give them that. its grown up hundreds of healthy happy ducklings and rescued ducklings for me. They grow at a normal rate with literally not a single one of hundreds or thousands (Ive not stopped to count) of ducklings ever having niacin leg problems, or later on, angel wing. And they just LOVE the stuff. When they are q couple of weeks or so, i also will give them a fee green peas as treats (we all know green peas = crack for ducks, lol) but only a small amount. Also other treats include some grass chopped into very small (like half a centimetre long) pieces in their water. And other vegies like carrots, beetroot, lettuce, kale, dandelion also chopped into very small pieces are great treats they love to have in their water.