Koi Parasites
Diseases and parasites of koi and goldfish are discussed in detail in a great book called Koi Health and Disease v2 by Dr. Erik Johnson. Do yourself a favor and get a copy. For the purposes of this website, I’m going to give you an overview of the areas you should be most familiar with.
Common Koi Parasites
IchWhite spots, flashing, heavy slime, fast breathing.
FlukesFlashing, clamped fins, ulcers after stress.
CostiaGray patches, lethargy, rapid decline.
ChilodonellaCloudy skin, flashing, respiratory stress.
TrichodinaFlashing, irritation, excess slime.
Fish Lice (Argulus)Visible parasites, jumping, ulcers.
Anchor Worm (Lernaea)Worm-like protrusions, sores.
DiflubenzuronCrustacean parasites: fish lice, anchor worm, gill maggots.
You might also value this discussion of Koi Viruses.
You might also value this discussion of Bacterial infections.