The first issue identified clearly on the Ranchu were flukes. Both skin and gill flukes were identified following scrapes and a straight forward Praziquantel course of treatment appears to have successfully removed them.
I have also identified what i'm confident is Costia in the pond that salt at low levels (0.3%) is having no effect on. Somewhat reassuringly, I have not (yet) found any Costia on the Ranchu themselves although I expect that they will be there somewhere and when confirmed they will be treated.
To start with, I will be keeping the Ranchu in alternative temporary accommodation for a few days whilst the main pond, heaters, air stones etc, are disinfected. With the Ranchu themselves not currently showing any obvious parasites on scrapes, I am keen to avoid subjecting them to any chemicals, if at all possible. That said, the Ranchu will continue to be scraped before being returned to the treated pond.