White Cloud Mountain Minnow... one won't close it's mouth ...?

We've got 3 white cloud mountain minnows and 1 goldfish in a 5 gallon tank (I know their supposed to be in 10 gal but somehow we forgot we only had a 5 when we bought them...) we've had the white clouds a few days and now one of them won't close it's mouth. I've never seen this behavior before, although we are somewhat new to fish. He also is acting kind of lethargic and spends a good amount of time near the surface of the tank and when we tried to feed him he wouldn't eat. We only tried putting food near him once because we don't want the other fish to eat too much. The other two seem to be doing great. Any thoughts? Thank you!