My duck is having trouble with his/her leg

So about a month and a half ago my parents got me two ducklings and they were doing just fine. Until two weeks ago one of them (the one we guess is older cause he/she was always a bit bigger) started not wanting to run or stand that much. We always joked and said that he/she was too fat and lazy.
That was until we noticed he/she had a limp. It wasn't bad at first but the past three days it had gotten worse. *two days ago it was when when we noticed it was hard for him/her to stand,one day ago it got worse, today he/she cant even walk or stand without falling forward*
I'm very worried because I've become pretty attached to them. They'd follow me around the house all the time and now the one won't leave the hurt ones side.
I've read some posts on here and they say some similar situations and what to give them. I plan on giving him/her greens, yeast tablets in the water, and Arnica Flour mixed with water to apply to his/her leg.
(I'm pretty sure it may be a sprain cause we have wood floors and they slip sometimes when running. But it would still help if someone could reply cause I'm new with ducks)