J & K: Pond keeping J to K … Jumping to Koi


An activity that koi keepers would rather not see in their koi. Very rarely an indicator of koi being full of the joys of life but more likely, unfortunately a behaviour borne out of irritation.

If jumping is a symptom of irritation, then it will usually be associated with scratching and flashing activity in other koi rather than being restricted to an individual fish.

Irritations can be caused by excessive numbers of external parasites, attached to the skin or gills by their irritating hooks or other modes of attachment. It is possible to verify parasites as the cause by carrying out a skin or mucus scrape and looking for moving parasites under the microscope. If a positive identification is made, then proprietary medications are available to satisfactorily treat the problem.

However, if parasites are not evidently the cause after inspection, then it is likely that fish are experiencing an irritation from poor water quality, with nitrite being a common offender.

The usual remedy of stopping feeding, carrying out a partial water change until the nitrite reading drops while investigating the cause should reduce the jumping symptoms.

Aquatic retailers are often faced with a dilemma when dealing with newly imported koi and pond fish as they especially have a tendency to jump when first introduced to new tanks and a change in water quality. This kamikaze behaviour subsides over a number of days once the fish have become more acclimatised and settled in their new environment. The only way of preventing koi from damaging themselves in this situation is to keep a net tightly covering the tanks.

Juvenile Koi

The majority of koi that are bought and sold each year in the UK are juveniles. In countries such as Japan and Israel, where most koi originate, koi will mature in their second or third season. If 1 year old koi are purchased and kept in the colder UK climate, then koi may take a year longer until they are producing eggs or sperm.

Buying juvenile koi is a cost-effective way of buying a handful of koi with potential, tracking their progression from juveniles through to mature specimens. It can be an interesting project to record their progress photographically, noting changes in their pattern as they develop and mature. Producing a pictorial montage of how specific koi have changed over the months and years can stimulate great conversations with other koi keepers or even with friends who don’t keep koi. A great way of introducing new comers to the hobby.


KH is usually one of the test kits that is last to be bought, after pH, ammonia, nitrite and nitrate. This is unfortunate as a KH reading can be extremely informative as it can be interpreted to relate to more than one characteristic of the pond water. Testing for KH must not be considered as an optional extra

KH is a German scale described as carbonate hardness. However, it largely measures the soluble bicarbonate ions which have a very close relationship with the pH (acidity/alkalinity) of the water.

Therefore, by measuring the KH, not only can you suggest the pH of the water but also the stability of that pH. i.e. Is that pH likely to change?

If water has a high KH, then it will have an alkaline and stable pH, just what is ideal for koi. However, if the pH is acceptable, but the KH is low, then the pH will be unstable with a tendency to drop below the desirable alkaline levels. In this particular case, it would be possible to fall into a false sense of security if only the pH had been tested without testing for KH. This would have indicated a healthy pH but without the warning of it being likely to drop.

Water with a high KH is referred to as being well buffered, as it resists swings in pH. The addition of calcium carbonate to the pond water increases the buffering capacity of water as it will dissolve whenever it is required. (see Limestone).


In a hobby that prioritises the science of filtration, as a means of providing a healthy environment for koi, the fish themselves have their own very effective filtration system- the kidney.

Kidneys are responsible for filtering out dissolved impurities from the blood on a continuous basis, preventing the blood from becoming toxic.

Like all filters, the kidney has a site of input and output. The blood divides into very fine capillaries, which intertwine with collecting kidney tubules which collect the impurities filtered out of the blood. The tubules later combine to form a larger collecting tube called the urethra which takes the urine from the kidneys to the vent area.

Water continuously enters a koi’s body in an uncontrollable manner and unless regulated, would cause the fish stress and eventual death by diluting the fish’s tissues.

It is the kidneys’ role to remove this water from a koi’s tissues, producing large quantities of a very dilute urine. The kidneys are called upon to work overtime when a koi is carrying a wound or an ulcer as excessive water freely enters the tissues through the opening. As this water influx is greater than normal, the kidneys must work overtime to maintain the correct water balance within the fish’s tissues.

Consequently, it can often be a combination of blood poisoning and kidney failure that cause the death of a severely ulcerated koi.


Koi is short for Nishikigoi, the Japanese for ‘coloured carp’. Koi on its own simply means carp and when describing these beautiful fish, simply ‘koi’ will suffice. The term ‘koi carp’ is often heard in koi keeping circles but this is an inaccurate anglicised term that literally translates as ‘carp carp’.

Koi have been selectively bred from a black carp variety called Magoi, over several hundred years and the koi phenomenon exists today as a result of the specific genetic characteristics of these fish.

If it was not for the complex genetic makeup of koi, then we would not be spoilt by the myriad of koi varieties and colour variations that we continue to see each year. The genes are responsible for koi breeding being a combination of an art and science, and an unpredictable, non-exact science at that. Even if the same broodfish are used several spawns in succession, the genetic roulette wheel will cause completely different koi to be produced and in different quantities.

The interaction of these genes over the many generations has enabled very gifted and dedicated breeders to select for certain features and traits. Japanese koi breeders will generally specialise, becoming renowned for one variety. The wonder and excitement of discovering what hand mother nature has dealt the experienced breeders after each spawn continues to motivate them towards what cannot be achieved, the perfect koi.

If koi were not such complex genetic animals, then it is fair to say that the intrigue and interest that exists in the hobby today would not be self-perpetuating, but would soon fade away.

The motivation of koi-hunting, visiting and re-visiting koi dealers to be amazed by the continual and seemingly endless supply of variations within one variety would not exist

Koi keeping has enabled hobbyists to develop their interest in these fish to many other areas such as filtration, nutrition, water quality and koi health. The opportunity of being able to undertake design, planning, construction, DIY and to discover and apply biology, chemistry and physics also makes this hobby so challenging and appealing.

Arguably, more time, effort and money is spent on these other facets of koi keeping, where new ideas and products can stimulate serious debate.

However familiar and confident we may become about the ancillary side of koi keeping, there is no danger of a koi enthusiast becoming stagnated as the genes continue to produce a never ending line of koi variations.

Kill blanketweed and string algae.