Koi keeping myths and fallacies popular folklore about keeping koi!

Different answers

If you ask six people the same question about koi keeping you will get six different answers. Sorting out the facts from the fallacies can be tricky!

Beginner beware!

In the equipment list I detail some basic equipment but I did not include two most important requirements for successful koi keeping: knowledge and experience. Koi keeping has a steep learning curve and maintaining a healthy environment for such large fish requires a certain amount of skill. All hobbyists need some basic knowledge of fish health so that common problems are recognized and dealt with correctly at an early stage, although the real skill lies in preventing problems in the first place.

Fish-health is an immense subject that embraces water quality, nutrition, filtration, fish physiology, disease and so on. Hobbyists need only a basic understanding but it pays to be aware that fish health is a science, competence in which is not achieved through personal experience alone. Sounds obvious? However, the beginner will soon find that koi-keeping is a hobby with many would-be fish health experts and this has led to a good number of old wives’ tales and fallacies which can mislead beginners. Let me address some of the most common ones. (There are many others!)

Ignorance the killer

Most health problems are related to water quality or other environmental factors, which can be avoided by proper  care and pond husbandry. The root cause of most  health problems is poor fish-keeping, not parasites and bacteria! This commonly happens when stocking levels exceed the owner’s knowledge and husbandry skills. As stocking levels increase so does the potential for problems, which can arise suddenly with devastating consequences, often resulting in large fish kills. So the first stage is to he honest and realistic about your ability to manage a heavily stocked pond. The next stage is to get a good book on fish-keeping and READ IT. (KoiVet has published 2 books so far)

Jumping for joy?

Fish often rub against solid objects and leap out of the water. It is possible that they do this to catch flies or get food or they may be playing. However, in the majority of cases it is because something is irritating them. This may be a minor irritation – similar to an itch in humans, perhaps – but it could be more serious. Whether you should act or not depends on the number affected, the frequency of rubbing and leaping, and the vigour involved. If one or two are having a lazy rub on the bottom drain there is no cause for concern, this is fairly normal. But if they are continually tearing about, rubbing vigorously against the sides or bottom or leaping out of the water, then further investigation must he carried out.

Unfortunately, one of the most common responses to this type of behaviour is to suspect a parasite problem and an anti-parasite treatment is administered without further thought. But this cursory diagnosis (or guess) overlooks the fact that any irritant can cause this response, including poor water quality (a high nitrite level, for instance), whereupon using an anti-parasite treatment can only make matters worse.

Becoming a aquatic health expert involves more than knowing what ‘treatments’ to use; indeed, the latter is quite easy as medications are described in most hobbyist books. Many fish suffer or die every year through inaccurate diagnosis and inappropriate treatments.

Skin and fin damage

Frayed and damaged fins, raised scales and skin damage, such as reddening or open wounds, are often attributed to the fish cutting or damaging themselves on sharp objects in the pond. Although this can happen it is not common, spawning being the exception.

Believing this to be the cause, many aquarists leave the ‘wound’ to get better, but invariably it just gets worse. The commonest cause of this type of damage is bacterial infection. Such instances need immediate attention. If caught early enough they are relatively easy to treat but in too many cases they are overlooked or dismissed as minor problems so that treatment is often given too late.

Keeping bugs out

It should be appreciated that, with few exceptions, most of the bacteria and parasites that cause problems are always present in the pond. Nearly all fish carry small populations of parasites, in the same way that most cats and dogs have some fleas. And all ponds are teeming with opportunistic bacteria.

There is still a common misconception, however, that bugs have to be brought into the pond before problems can occur. So when problems do arise, new additions, frogs or birds are blamed. This is possible, of course, but most problems start in the pond, where deteriorating conditions allow resident bugs to gain the upper hand. Healthy fish can fight and control the number of bugs, in the same way that we humans control the bacteria and viruses that surround us. In most cases these bugs are only harmful to weakened or stressed fish. Constant parasite or bacterial problems usually indicate a system management problem

Preventative treatments

A common practice in koi-keeping – I am amazed at how common – is to give regular, often monthly, chemical treatments to control parasites and other bugs. While this sounds a good idea, in practice it serves no useful purpose and can be detrimental to both fish and filter. The reproductive rates of most bugs are quite phenomenal – particularly at summer temperatures – and even after chemical treatments they can quickly return to their previous population levels. It is important to realise that these bugs should be controlled by good fish-keeping practices, not by chemicals, many of which are toxic to fish.

If there is a continued problem with parasites or bacteria, it indicates a more fundamental problem – usually environmental. Chemical treatments are a last resort, not the first, and should only be used when there is a clearly identified problem. Do not fall into the bad habit of trying to fix a problem that doesn’t exist. Many koi-keepers do.


This has been a brief insight into some of the more common misconceptions that the beginner to the hobby may be exposed to. Fish, like most animals, are creatures of habit and any strange behaviour or change in appearance can indicate a potential problem. Being observant and noticing these changes, or clinical signs, is the first step towards truly successful healthy koi-keeping.

You might have the impression by now that proper koi-keeping is an almost impossible task, especially if you don’t have an arsenal of chemical treatments to hand. What I have tried to say is that koi-keeping is only simple once you’ve learnt to dispense with the chemicals. The fish, believe it or not, are fully able to cope with most potential health problems once they have been provided with a good diet and a healthy environment.