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In every pond there are organisms which can cause disease and in a well-managed pond the fish do not succumb to clinical disease because of their natural defences. This situation can be thought of as a balance between the bugs or organisms in the water and the fishes' natural defences, a delicate balance because a koi pond is a small and closed volume of water. i.e. the water is recycled rather than replaced.

The physical defences of a fish comprise the skin and mucous layer and these may be compromised by physical trauma e.g. the direct physical damage caused by anchor worms or chemical trauma e.g. chemical treatments, ammonia, nitrite and abnormal pH. In addition, a fish has an immune system. comprising cells and antibodies, and they can be compromised by stress, low temperatures (say. below 10 C). poor general body condition. breeding condition or lack of previous exposure to a disease-causing agent the sort of thing that vaccination helps protect against because it primes the immune system by introducing low-level exposure to a disease.

The ideal is to have no disease-causing organisms in the pond but in practice this is impossible. However, with good management practice the number of such pathogens can be controlled to a level at which the fishes' immune system can cope adequately. In this way, fish manage to live healthily, despite the omnipresence of disease-causing organisms.

If disease-causing organisms are allowed to multiply (through poor environmental conditions) or the fish is compromised in some way then its defences become inadequate and clinical disease may develop. Koi have skin and a mucous layer for its physical defence. and an immune system for its biological defence.

Achieving the right balance: The number of organisms may be decreased by:

• · Dilution-with large volumes of water
• · Clean water : which avoids the conditions necessary for bugs to thrive
• · Healthy fish, which are better able to defend themselves against disease
• · Protection from disease-carriers
• · Lower temperatures

• The number of organisms may be increased by:

• · Carrier fish (e.g. brought in by new and inadequately quarantined fish)
• · Stress, which allows organisms to breach the fishes' defences and thrive
• · Rising temperature (e.g. in spring)
• · High stocking densities (le too many fish)
• · Introduction of contaminated plants
• · High organic loading caused by dirty pond water