I'm not sure why you're treating a fish when you really don't know whats wrong with it.
My guess would be stress from being in a tank with other fish.
Before medicating her why not try her in a heated filtered cycled 5g tank and see if she dosn't come around. .5g isn't enough to see a difference in her. It can't be heated properly and
ammonia can build up quickly.
If you're set on medicating her, it's always best to only medicate the fish that's sick and not expose healthy fish to medications.
Edit: Interesting how my response to the original question doesn't make sense now that the question has been edited.