[Peter Finke]

Your question cannot be answered with certainty. There could be different causes and consequences. Let me tell you one that is likely.
The scratching could well be caused by Oodinium. Then, all fish are highly dangered. For a success by separation it is probably too late.
A very effective medicine is called “2-amino-5-nitrothiazole”. It is widely used by veterinary doctors as a desinfection means. A yellow powder. It must be stored in the refrigirator, otherwise it will turn brown in some months and get useless.