As a Koi hobbyist, you should be aware of the common diseases that can affect your Koi, as well as recognize how they show themselves and what you need to treat these diseases. There are more than 100 known diseases and parasites that can infect most species of pond fish and Koi.

Koi Diseases are the result of interaction between a host, a pathogen (a disease causing agent, like a bacteria, parasite, or virus), and the environment. Diseases in Koi can be infectious or noninfectious. Infectious diseases are caused by microorganisms that challenge the fish’s defense mechanisms by invading, infecting, and disrupting the normal functioning of the host. Noninfectious diseases are caused by factors such as environmental conditions, nutritional deficiencies, or toxins.

Koi do not jump for happiness, they jump to get more oxygen into their gills, if they have parasites or gill filament damage. Koi will also jump when the pond has too high TDS levels, lots of heavy metals or other irritating materials in the water that make it undesirable to live in. Koi hate copper in their water and will sometimes swim erratically when levels reach toxic range. Even chlorine can burn the sensitive gill filaments and tissues, making it difficult for the Koi to breathe.

Koi produce excess mucus to combat problems. It's like a natural defense they have, so when you see Koi producing excess mucus it's a sure sign your Koi are stressed.

Anchor Worm is a tiny thread-like crustacean that buries itself under the scales of Koi and then enters the dermis. Heavy infestations can severely deplete a Koi's strength and lay it open to secondary infection.
You will notice flashing and rubbing as signs your Koi has Anchor Worm.

The Mycobacteria are the smallest organisms that are capable of self-replication. Unlike other bacteria, they lack a cell wall.
A zoonotic potential exists with mycobacteria, and people handling infected fish can get lesions on their hands from contamination of cuts or scrapes. This is known as “fish handler’s disease”.

Like carp pox, this Koi disease is an unspecific condition in which the eye appears to be covered by an opaque film. It is thought to be bacterial and due to incorrect water conditions, but may also be linked to other identifiable Koi diseases. The addition of pond salt to the water may prevent the condition.

Lice will irritate your fish, with hooks on the legs and the stinger that injects toxins and withdraws fluids. This constant piercing of the skin causes inflammation. However, the biggest threat is, as the fish lice move from one location to another they leave a hole in the protective "slime coat" and skin. These "holes" give the opportunistic bacteria such as Aeromonas or Pseudomonas which are one of the biggest causes of deadly ulcers (body sores) to invade your koi and pond fish.