Anyone would need more info to help you Adam... A good start would be to fill out the disease form...
A few things come to mind for me. How long are the others in qt, the same 3 weeks?
How long have you had the first 9? Were they all added at once or mixed different with no qt?
Are you opposed to possibly treating your entire 125, good chance if one is showing signs others may. Taking out the 1 bad one adding new ones may help the bad one, may not, but I think you'd do more harm and have more issues this route.
No other of the 9 show bad signs but this one?
Others can chime in but I think it may be best to deal w the entire 125, or drain it half down and possible treat, or even try some salt to start.