About injecting the Koi of the gills diseases. You should avoid injection as much as possible. About the gills symptom. When gills cell melts, you must not do injection. It will be early by doing injection in this case to die sometimes.
Injection is OK when gills cell exists.
Injection will be effective only in the early grade of the gills diseases. About the method which does injection. It decides Koi to be killed by that do to do anesthesia. You must not use anesthesia. The veterinarian who doesn't know this, and doctor will be murder Kois.