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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


Our hobby is all about keeping healthy koi. Sick or, worse, dying fish cause almost as much distress to the keeper as to the koi themselves. Unfortunately it is also true that it is very difficult to cure many ailments so the first lesson is prevention.

  • The truth is there is no great secret to healthy koi

  • Most of us know the golden rules – it's just that we don't keep to them

  • Clean unpolluted water

  • Plenty of oxygen

  • No debris in the pond or the filters



Our members keep a variety of styles of pond ranging from the ultimate hi-tech heated covered palace to the more natural style garden pond. However experience has shown that to keep Koi succcessfully there are some golden rules that cannot be broken.

  • · Koi are large fish – they need plenty of space

  • · Extreme temperatures are to be avoided, so try to design your pond to enable it to be covered in winter

  • · Koi like deep water – try to keep a minimum depth of 5 feet

  • · You will need to catch your fish from time to time – make sure you can reach all of the pond

  • · Koi need clean pure water

  • · It is best to fit a bottom drain to enable water to be taken from the bottom of the pond straight to your filters

  • · Koi eat a lot – you will need large biological filters to keep the water sweet

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