I would also add some no-flush, no slow acting niacin to their water. Maybe give them some chick electrolytes for about 2-3 weeks and see if the leg problem doesn't improve with that.
It could be as simple as a supplement deficiency and it won't do any harm to them if you do add it to their water to see if that helps. You should maybe repeat the niacin (I would go with a high dose of 500mg/gal water) once every other month for a few days. Again, it
Doesn't hurt or harm them but vitamins and supplements like niacin is also usually in their food.
My one duckling actually was to the point to almost not walking anymore last year. Now he is for and healthy, and a bit spoiled by mama too. Lol