Latest Updates



Click for seminar details


KOI logo (135)
Who are we?




Food for thought (3)       (Food for thought 3)


Stocking levels
A stocking density as low as one large koi per 1,000 gallons is sometimes recommended but what does that mean? Is a 2,000 gallon pond really only suitable for two koi?  Since koi prefer to shoal together, putting just two koi in any pond would be unkind. Even in larger ponds, stating how many koi they could hold is meaningless without specifying their size.

An average 20 cm koi weighs around 130 gm. If it doubles its length to 40 cm, it will weigh over 1 kg.  When it has grown to 60 cm it should weigh in excess of 3.6 kg – 27 times more than when it measured 20 cm although it has only increased its length by three times. Stocking densities are limited so that the capability of the filter isn’t exceeded but the pollution produced by koi is neither a fixed amount nor related to length, it is determined by body mass.

As an example; for a 3,000 gallon pond, if its filters could comfortably cope with the waste from three 60 cm koi weighing a total of about 10 kg, that same system could contain 81 koi of 20 cm each because their combined weight would be the same.  That density seems uncomfortably high to me because it doesn’t take into account the fact that a 20 cm koi can grow quickly, so for this example to work there would have to be a continuous process of thinning out and moving on. But if the 81 were reduced to 54 by the time they had grown to 40 cm and then to just three by the time they had reached 60 cm, at no time would the filter have been overloaded.  It would only have been dealing with about 10 kg of koi throughout.

For optimum growth, 4 kg of koi per 1,000 gallons is a realistic amount, and a close approximation for a koi’s weight can be found from the formula; length3 divided by 60 = weight. For non-mathematicians, the easy way to work that out is to measure each fish’s length in cm then, on a calculator, press length x length x length divided by 60.  This gives the weight in grams for that fish. If the total of all the weights doesn’t exceed 4 kg per 1,000 gallons, and the filter system is good, then there is the potential for good growth.  That density can be exceeded if the filtration and turn-over are up-rated accordingly but growth rates may be reduced.

In theory, with modern filter technology, there isn’t an upper limit to stocking density until the pond looks seriously overcrowded. For any stocking density, as long as there is sufficient aeration and a filter system capable of keeping the other water parameters at optimum values, it could be argued that the fish have a good environment.  But to look at water quality alone as the sole indicator of a good environment would be like saying that the living conditions of battery hens are as good as those of free-range chickens simply because all their basic needs are being met.  When fish are unhappy with their surroundings, they release into their bloodstream stress hormones such as cortisol and epinephrine.  Overcrowding is one factor that causes elevated levels of these hormones.  Apart from the effect on growth, should we also consider the “happiness” of our pets? I think we should.

Opinions vary but, if 4 kg of koi per 1,000 gallons will allow maximum growth, I would say that even if growth isn’t the prime objective, doubling this to 8 kg per 1,000 gallons is the maximum stocking density for happy koi.

Back to topic list


Ozone has been used to sterilise water in swimming pools for many years because it is very corrosive and has a devastating effect on harmful bacteria that would cause cross infection from one bather to another. Manufacturers and suppliers of pond equipment are always looking for new products to sell to pond keepers and so it was only a matter of time before they made lower power ozone generators for koi ponds. The principle of operation is simple.

Single atoms of oxygen (O) do not like being on their own so they join up in pairs, which is why we refer to oxygen as O2. In an ozone generator, an electrical arc or UV lamp splits up some of the pairs and forces them to join up in threes (O3) but this is an unstable relationship. So, the third atom soon breaks away and looks for a new partner. If it finds another single oxygen atom, it makes a new pair (O2) and is no longer corrosive but if it should bump into an organic cell it will "burn" its way in through the cell wall and, in doing so, the cell will be fatally damaged by a process called lysis.   This is most easily described by imagining the cell as being similar to a balloon half filled with liquid.  If the balloon is punctured, the contents will leak out.  In the case of an organic cell, as its contents leak away, it rapidly dies.  Bacteria are single cell organisms, so if their only cell is damaged then they will be killed almost immediately on contact with water containing ozone.

But ozone is indiscriminate. It doesn’t only kill harmful bacteria, it will destroy any living cell it meets – good or bad.  This is why, when ozone is used to sterilise swimming pools, the water has to be drawn water from the pool, sterilised in specially designed equipment and then, to prevent its corrosive effect on bathers, all trace of ozone must be removed before the water is returned to the pool.

In the case of koi ponds, if water is drawn from the pond, sterilised in external equipment, and then any free oxygen atoms are removed before the water is returned to the pond, then I would be happy to recommend its use.  But this equipment has to be installed and maintained properly. It is not a handy add-on that can be plumbed into a pond without full knowledge of how it works and the risks involved.  Other types of ozone generators that are designed to inject ozone directly into water returning to a pond are a different matter.  Fortunately, all these types of units that I’ve seen are very low power.  Their ozone output is low which means that the corrosive effects on koi and good bacteria in the biological filter are not great.  The koi self repair system replaces damaged cells as quickly as they are being damaged and beneficial bacteria can easily multiply to replace any bacteria killed by ozone circulating through the bio-filter.  But if direct injection ozone systems cannot cause enough damage to a colony of filter bacteria to wipe it out, the question I would ask is can they really have a significant effect on any harmful bacteria that might be in the pond?

Back to topic list


Koi predators
Evolution has ensured that any species of animal which is a natural prey for a predator will have evolved a colouration that helps it blend in to its background, making it less easy to spot. The original Magoi from which koi as we know them today were bred, were dark and inconspicuous but selective breeding has produced the brightly coloured koi we now keep in our ponds. This makes them a highly visible target for predators. Arguably the most common predator that a koi keeper has to guard their pond against is a heron. British herons are not migratory and so they can attack a pond at any time of the year and they are especially active when they have young to feed during their breeding season which starts as early as February.

They feed on many different animals that live in or around natural waterways, including frogs, water voles and even young birds but their most common food is fish which they hunt by standing motionless, either at the water’s edge or after having waded into shallow water. When a fish swims by they will stab at it, possibly several times, to disable it before grasping it in their beak. This means that when a heron targets a koi pond, even the ones that it doesn’t quite succeed in catching and eating may be scarred or badly injured.   And it isn’t only those small enough for it to swallow that are at risk, if a heron can succeed in disabling and grasping larger fish it will pull them out of the water and dismember them.

There are two common beliefs about herons. The first is that, because they are a solitary bird, placing a plastic heron by the pond will be a good deterrent because a real heron flying over will assume the spot is already taken. The second is that herons don’t land on water.  But two stories show that these beliefs aren’t correct. On amusing story tells of a koi keeper who bought a plastic heron as a deterrent and said that it was now being used by a real heron as a perch to provide a vantage point to look for an opportunity to get at his koi. Another story tells of a heron that was seen to land on a lake, float for a while, then stab and catch a fish and take off again. The landing and take-off were described as very ungainly but the heron was successful in catching its dinner.

So if we discard the idea that a plastic heron will keep our koi safe and also bear in mind that a heron can land in water, what can be done to protect them?

Nets are one obvious method but they can spoil the look of a pond and make maintenance difficult.  A good choice that seems to be successful and is less visually intrusive is a strong fishing line or electric fence around the pond. If a few strands of fishing line are stretched over the pond so that a heron can’t land directly in the water, protection will be complete.

Movement activated water jets are another possibility but if you use one, make sure that the water supply to them can be turned off before approaching the pond - they don’t distinguish between herons and humans.

Back to topic list


Vegetable filters
Koi keepers often refer to “the nitrogen cycle” but in fact, in most koi ponds, the process isn’t really a cycle at all. Ammonia, nitrite and nitrate are all different compounds that contain nitrogen. In the full nitrogen cycle, the nitrogen in ammonia excreted by fish is converted by bugs in the bio-filter, first into nitrite and then into nitrate. That is where we normally leave it with the nitrate slowly accumulating in our ponds and with only water changes or filters having special denitrifying characteristics to reduce the level.  But, in natural waterways, the process continues with the nitrogen in nitrate then being used as food by plants which, in turn, are eaten by fish resulting in them excreting more ammonia so the cycle is completed and repeats endlessly. A koi pond with a vegetable filter is far nearer to the natural nitrogen cycle, especially if watercress is grown in it, because watercress can be fed to koi as a source of vegetable protein and minerals that may not be in their normal food.

Plants “prefer” ammonia as an energy source. This is because ammonia contains just as much of the nitrogen that they need as does nitrate but also, because ammonia is such a small molecule in comparison with nitrate, it is more easily assimilated by plant roots. In addition, taking up nitrate actually costs plants energy because, unlike ammonia, nitrate has an electrical charge that has to be overcome first.  The energy required is so substantial for the plant that nitrate is only taken up in daylight when energy can be obtained during photosynthesis, whereas ammonia is so easily absorbed by plant roots that it can be taken up even in the dark. In addition to ammonia and nitrate, plants in a vegetable filter also take phosphate out of the pond water.

A simple vegetable filter consists of a bed of coarse gravel with water flowing into it at one end to form a shallow pond which then overflows back to the main pond via a waterfall at the other.  Any non-poisonous aquatic plant that grows vigorously can be grown in the gravel. My personal favourite is watercress because gram for gram, watercress contains more vitamin C than oranges, more calcium than milk and more iron than spinach. It is also full of beta-carotene and vitamin A equivalents.  Plants that have reached their full size and stopped growing need less nutrients so, to keep it growing vigorously, watercress should be regularly cut back hard and since it is nutritious for koi to eat, bunches of the trimmings can be thrown into the pond for them to enjoy.  This will add a little nitrogen back into the pond but when the rest of the trimmings are put onto the compost heap they will take with them a great deal of the ammonia, nitrate and phosphate that they have removed from the water.

Back to topic list


Japanese koi versus British
I personally couldn’t tell the difference between a good quality Japanese koi and an equivalent quality UK bred koi, and I would doubt that anyone could say for certain that one fish is Japanese and another is from UK stock just by comparing them against each other.

If that is true, why do British koi keepers go to Japan to buy koi rather than the other way round, with Japanese koi keepers visiting this country to take home koi from British breeders? Genetics plays a significant part in the development of koi from spawning to maturity and genetics is purely and simply a numbers game. As we grow from birth to adulthood, we are rarely identical copies of our parents but we are likely to resemble them.  Our children can expect to resemble us and so family traits will be passed on through the generations but, over time, these characteristics change and evolve.

So it is with intensively bred fish such as koi but with one exception.  In Japan, once a particularly desirable characteristic has been spotted by a breeder, they will selectively breed until that characteristic becomes the norm for that particular bloodline.  One breeder may specialise in one variety or trait within that variety, another breeder will specialise in something different.  Japanese breeders can do this because koi breeding is well established over there and breeders, not only have access to a huge pool of koi to choose from, but also they have accumulated wisdom passed down through the generations.  This helps them to predict the numbers game of genetics.  They know what is likely to produce the most successful outcome and selectively breed accordingly.  Of course, predicting genetic characteristics is not an exact science.  No doubt they have many failures where a particular spawning may not produce the desired outcome but the numbers game comes into play again.  The sheer number of breeders and the brood stock that is available for them to choose from means that, overall, the chances of successful outcomes are far higher than failures. Selective culling does the rest, resulting in Japan having earned a reputation for breeding very high quality koi.

Does this mean that British breeders are Japan’s poor cousins producing inferior quality koi?  I would argue that they are not.  Although the numbers game for producing the highest quality koi is heavily weighted in favour of Japan, British bred koi win prizes at British shows when competing against Japanese koi so they certainly can’t be called “inferior”.  Will a British koi ever win the All Japan Show?  Well, they already regularly win first prizes at koi shows over here and you have to start somewhere!

Back to topic list


Filter maturation
Biological filters are not the only place in the koi pond and filter system where biological filtration takes place. Nitrifying bugs (nitrosomonas and nitrobacter) are prolific in nature, on land, everywhere in the aquatic environment, even in the air but it is only how they behave under water that is of interest to koi-keepers.

In a koi pond, they colonise any wet surface including the floor, walls and even inside pipe-work. In fact, they prefer the inside of pipe-work to open surfaces because they are photophobic, which means that they don’t like light. Light doesn’t harm them, but they prefer dark places and will multiply better without light.  These bugs can swim because they have little whip-like tails that they can rotate and propel themselves through water. They can also take part in the nitrogen cycle whilst swimming but are more efficient when they can form a biofilm on solid surfaces.  They clearly can’t think or plan where they are going but they will propel themselves around seeking to avoid the light until they chance upon a dark place and settle onto a solid surface where they form their colonies.  Ensuring our bio-filters are dark is the first step in encouraging bacteria to settle there and grow into mature colonies.

It is often thought that these bugs use either ammonia, (in the case of nitrosomonas) or nitrite (in the case of nitrobacter) as a food but, in fact, what really happens is that they break apart these chemicals and this releases a form of chemical energy which allows them to capture carbon from carbonates in the water which is what they really need in order to grow and multiply.  For a bio-filter to be able to support a mature bacterial colony there must be a plentiful supply of carbonates.  Ensuring that the KH is sufficiently high is essential. A bacterial colony cannot mature without carbonates and even a mature one will not remove ammonia from the water without a supply of these.

As well as a form of carbonate in the water it is vital that the water flowing through the bio-filter is also well oxygenated, in processing ammonia through nitrite to nitrate, bio-filter bugs use over four times as much oxygen and over seven times as much carbonate as the ammonia they process.

If all these conditions are in place, the bio-filter will quickly mature from naturally occurring bugs without external aids such as seeding with a filter booster or starter.

Once a filter has matured, there is a common problem experienced each spring in unheated koi ponds.  Filter bacteria don’t die during winter but become less active.  As water temperatures fall to around 5°C they protect themselves by going into a dormant stage and shutting down their internal biochemistry in a way that allows it to restart when the water warms again. Once shut down, they are effectively in a state of suspended animation and cannot restart activity until the temperature rises significantly.

Koi begin feeding and excreting ammonia before this temperature is reached resulting in a springtime ammonia spike.  There is little koi-keepers can do during these couple of weeks except keep feeding to a minimum so as to keep the ammonia spike below about 0.5 mg/L.  If this level isn’t exceed, the koi will come to no harm.

Back to topic list