Not sure how your heat source is in your brooder. Usually the heat source should be at one end, or center, and have enough room inside so chics can wander away to area that is just the correct comfort for them. When they are cold, then they move closer to heat source. When too warm, they walk...
Keep her safe by possibly separating within same brooder, and feed her some Boiled egg yolk, as well as some of the egg-white. The yolk has more nutrition that may get lil chickie on the road to recovery and regular growth. I have no suggestions on any additives into water. No experience...