I agree with crypto and before you can decide best treatment, you need to be fairly certain what is the infectious agent of the fish. Some of the signs of infection with Cryptocaryon irritans are rubbing or scratching against decorations or substrate (this behavior is also known as glancing or flashing), breathing problems, an increased mucous layer, loss of appetite, abnormal swimming behavior, frayed fins, cloudy eyes, and, of course, the telltale white spots. These characteristic spots are usually described as appearing like small grains of salt stuck to the body of the fish. Even in the presence of all of these external signs, the best diagnostic tool is microscopic examination of fresh fin or gill clippings or skin scrapings. But, realistically, an extremely small number of us are ever going to perform this kind of differential diagnosis, myself included.
I’m surprised after alleged qt that this would occur. Copper is a highly effective medication against Cryptocaryon irritans when dosed and maintained in the proper concentration.. always have a test kit when dosing copper.
Other option is formalin (general cure is one form)
Formalin can be administered one of two ways; either in short dips with saltwater or used continually in a hospital tank. The dosage for the continuous use is 1 ml of the 37% stock solution for every 25 gallons of quarantine tank water . I prefer the formalin dip to continuous use because formalin is a fairly toxic compound. Also, with no commercially available test kits to monitor the concentration, it would be difficult to dose an entire tank and account for evaporation, absorption, etc.
To prepare the dip, Itake 5 gallons of tank water and add to it 3.75 ml of 37% formalin. also aerate the water vigorously to ensure there is maximum dissolved oxygen. The dip should last 30 to 60 minutes.
