Treating Infections and Ulcers in Koi and Goldfish
Besides discovering and correcting environmental problems with the situation, attacking bacterial infections in Koi and Goldfish is a large undertaking. Especially if the infection is impacting the ability of the fish to breathe: The GILLS.
The facilities can be very large as in the case of ponds. In fact, so large you can't actually HOLD the fish for treatment.
The following article discusses how to treat bacterial infections when you CAN hold the fish for treatment, and when you CANNOT hold the fish, also discusses what to do in colder water versus warmer water.
There are related articles, like how to SHOT GUN parasites in the scenarios with bacterial infections on your goldfish or Koi.
Always, a microscope is the best way to protect yourself from guessing.
Thank you for your kind attention!