The fish developed reddish lesions on the skin, and face. Some white slime was attendant at these lesions. Biopsies were taken and nothing was found. I asked if he'd biopsied the specific lesions themselves, and indeed he had not, because he felt it might further traumatize damaged tissues. This was a logical concern, to be sure, but as I stated before.
Costia can exist focally, in small patches that can be missed unless they are deliberately scraped.
Formalin is the best treatment for salt resistant Costia. Increase circulation with a floom or with a spraybar. Run this level of Formalin for two hours and then execute a forty to fifty percent water change, with de-chlorinator. Repeat the treatment in 72 hours. Do this Formalin treatment for a total of three treatments and you can rid your system of Costia and Flukes.