Home Koi For Sale Koi Care Koi Kam Plant Profiles Plant Tips Supplies
Addiction Shipping Our Ponds & Fish About Us
Water Quality Baby Koi Special How and When to Feed Koi Eggs For Sale Favorite Links Buy a Koi Cam Camera
Bonnie's Online Shop Join My Email List Table of Contents Search Our Site Contact Us

Alan Levington  (copied with permission)

Parasites in koi ponds can often cause serious problems for the koi, and keeper alike. By this statement I mean, that the fish have the parasite problem, and the keeper has the problem of curing it! Knowing which parasite or parasites are present is essential for the application of the correct remedy.

In order to be able to control levels of fish parasites in ponds, it is necessary to understand something of their life cycles. Understanding how they reproduce, and what their various life stages are, can sometimes allow us an opportunity to seriously reduce their numbers.

A microscope can assist in making a precise initial diagnosis of the problem, and as you will appreciate from the following text, further microscope tests are preferable in order to apply follow up treatments which are necessary with some parasites, such that the reproductive cycle of the parasite is broken. You may well appreciate that the subjects in the title of this article are closely linked.

Problems with parasites are usually associated with either new introductions to the pond or some environmental problem such as low oxygen levels, nitrite or high nitrate levels. Low oxygen levels can severely stress fish and can be caused in several ways.

Low oxygen levels are more likely to prevail in summertime, when temperatures are higher, although some treatments for parasites can target algae and pond debris as part of the chemical reaction aimed at the parasites, and in so doing, create oxygen shortages.

It is wise to increase aeration in the pond before, during and after any medications are used. As fish consume approximately four times more oxygen after feeding, it is best that food is withheld for the duration of any treatment.

Simply overfeeding the fish can cause nitrite levels to rise above normal levels, or it can happen when the stocking level is in excess of that which the biological filter can cope with. Low oxygen levels and low KH frequently lead to nitrite levels rising. The whole pond system relies upon oxygen to function and adequate resources of calcium carbonate are essential to the bacterial activity of the filter. In systems operating in areas where the water is soft, and winter and spring pond care is kept to a minimum in terms of water changes, the borderline level of calcium carbonate can be reached. If this happens, the increased activity within the pond can use up the remaining carbonates and biological filtration problems can occur.

Nitrate is the end product of the biological filter function and can be controlled by regular water changes. Test kits are available for measuring all the water parameters mentioned.

Frequent parasite problems would tend to suggest that the environmental factors within the pond are less than ideal. Fish living in a pond with good water parameters are able to cope with the small levels of parasites, which are often present in small numbers in perfectly healthy ponds.

It is difficult to take any one subject, which is part of the overall scene of keeping koi, without the subjects overlapping with other subjects relating to the koi pond.

Low oxygen levels and nitrite contributing to a parasite problem – never!! Or so you may have thought before reading the above facts.

Carp did not evolve naturally in Europe, due to this they have not fully adapted to climate here. They evolved where winter temperatures are 14C and summer temperatures can reach 25C. Due to this their immune systems have evolved to operate in this temperature range.

At low temperatures very little activity occurs in a pond, bacteria, parasites and fish are all relatively inactive. However once temperatures begin to rise all activity increases. This can cause problems for a koi carp because the increased bacteria and parasite activity are occurring before the koi’s immune system is able to react to this threat. Bearing this in mind all koi keepers should be particularly vigilant in the springtime.

Some keepers take the prophylactic approach: they rid the pond of any undue organic waste, and rid the koi of as many parasites as possible in the late autumn when temperatures are still above 15C.


Most parasites will cause fish to react in a similar fashion. Points to watch for would include the following:

Fish remains alone and ceases to be a sociable shoal fish.

Fish jumping or scraping against pond side and floor.

Fish refuses to feed.

Fish breathes heavily, opening and closing of the mouth and gills.

It is possible to have parasites in a pond and only one, or a few of the fish seem to be affected. As individuals they can differ in personality as well as their ability to resist disease and parasites.

If this is seen to happen, it is possible to treat an individual by way of a salt bath. Seawater contains about 3% salt. The bodies of freshwater parasites contain about 0.5% salt solution. Osmosis is a simple physics fact which guaranties that water will always tend to go towards an area of high salt concentration. In a salt bath of water of 3% solution, the parasites, being simpler creatures than the fish, can have the fluids withdrawn from their bodies by this difference in salt concentration. The same laws of osmosis exist for the fish so care and continuous observation of the fish during this treatment is essential. Once the fish begins to lie over on its side in a similar manner to that which occurs when a fish is anesthetized, it should be removed from this salt bath and placed into another container to recuperate. A maximum of two minutes is advised for this treatment which can be repeated if necessary a short time later.

An improvement in the disposition of a fish after this treatment can suggest that parasites may be the problem.

In the wild, fish can often rid themselves of parasites by changing where they live in a lake. River fish can have parasites washed downstream where they can no longer infect them. However, in a closed pond system, the detection of a parasitic infection is best dealt with by the appropriate treatment. Always check water parameters first if fish behave in an unusual manner.

Spring variations in temperature can stress fish. They have evolved to expect quite stable temperatures, which change very slowly with the seasons. Large bodies of water resist temperature changes much better than small ponds.

The first parasite to be discussed is "White spot". This parasite is frequently seen after sudden drops in water temperature.

One of the main reasons for failure to detect parasites on a skin scrape is incorrect preparation of the slide sample. The following tips should help in taking a sample of mucus from the fish and preparing the slide.

Net the fish and gently use the glass slide to remove a small amount of mucus from the surface of the skin. The term "skin scrape" is misleading as you are only taking a small sample of mucus which covers the skin.

Samples of mucus from the gills. Caution - the gills of a fish equate to the lungs of a terrestrial animal. Extreme care must be exercised whenever gill samples are taken.

Taking samples from the gills is a very delicate operation and many inexperienced koi keepers will avoid this task. Dead, whole fish are sold in most supermarkets and a practice run on one of these may help in training for the real job,

If a sample of mucus is required from the fish’s gill, the fish must first be anaesthetized. Once the fish is motionless the gill cover can be raised and the glass slide very carefully inserted between the layers of gill tissue to collect a sample of mucus. Scraping it together using a cover slip should collect this tiny quantity of mucus, which is on the glass slide. Add a small amount of water to the sample and cover with the cover slip. Gently press the cover slip onto the slide until the sample spreads outwards towards the cover slip edge. On small fish, a wet cotton bud or a small loop of wire may be used to gently remove a sample of mucus from the gill.

Always begin searching a slide with the microscope at the lowest power setting. This is usually 30/50X magnification. If the slide has been prepared correctly as above you will be looking at what is best described as an aerial view of a coastline with perhaps several islands lying off this "coast".

Any large parasites should be visible at this magnification. Typically, mature forms of "White spot" and gyrodactylis (skin fluke) would be visible.

A further search of the slide at a higher magnification of100X will reduce the amount of mucus sample visible through the microscope, as it appears to bring the sample closer to view.

Further explanations are given on identification as the various common parasites are discussed. It should be said that the major problem for anyone learning to use a microscope is what to ignore as "normal". Later explanations and photographs taken through the microscope should help with firm identification of individual parasites

Click to see microscopic photos of Ich   (photos done with microscope and digital camera)
One of the most common of all fish diseases is known as "White spot" or "Ich". The latter name is a shortened version of the name of the parasite "ichthyophthirius multifiliis".

The common name "White spot" was given by aquarium keepers due to the fact that badly infected tropical fish could be seen with white spots on their bodies. These white spots are rarely seen on koi unless the infection is severe and well advanced: suspicion of the disease is by characteristic changes in the koi’s behavior and diagnosis is confirmed by microscopic examination of skin/gills scrapes

Ich has an interesting life cycle, one stage of which is illustrated in Fig 1. At this stage of the life cycle it is quite easily seen on skin/gill scrapes, however at another of its life stages it bears no resemblance to Fig 1. Being comparatively small and of an entirely different shape, it is often not recognized.

Stage 1. For simplicity we will take the stage shown in Fig1 as the first stage of the life cycle of Ich. Once maturity has been reached at stage1. the parasite leaves the fish and drops to the pond floor where it covers itself with a protective coating and continues to develop.

Utilizing energy, which it has gained from feeding on the fish’s tissue, it divides into numerous individual "tomites" or baby "Ichs". From 500/1000 individual tomites can emerge from this one encapsulated individual. These are frightening numbers, but as this parasite has evolved to meet natural stocking levels, it produces these huge quantities to increase the chance of one individual finding a host and continuing the cycle, much the same as trees and flowers produce large quantities of seed on the off chance that one will find a suitable place to grow and perpetuate the tree or flower species.

On leaving the capsule the tomite becomes free swimming and searches for host. At this time it is extremely small. If a fish is not found within a short time the tomite will die. If it finds a fish it can use this as a food source and will grow once again into the relatively large, easily recognizable form of Ich, and so the cycle continues.

If the number of fish available for the tomites to colonize is large, there is obviously a much greater chance of them being successful in their attempts at finding a new host fish. In koi ponds we stock at much higher densities than would normally be found in natural environments, hence the risk of re infection is much greater A few parasites would cause little harm to a fish, but once numbers increase significantly the fish soon becomes distressed and if treatment is not applied quickly, serious skin and gill damage will occur which can kill the fish.

The preferred treatment for Ich is Malachite Green at 0.1 grams per ton (0.1gram/1000litres i.e. 0.1ppm) together with formalin at 15ml/ ton (15ml/ 1000 litres i.e. 15ppm). Commercial preparations of these chemicals are available and the instructions should be followed according to pond size.

This treatment will kill the parasite whilst it is in the free-swimming stage and on the fish, but when the parasite is in the encapsulated stage on the pond floor, it is protected from any know treatment that would kill it without affecting the fish.

Typically, if malachite green and formalin were used, this would kill of all visible stages of this parasites life cycle, and a microscopic examination following this treatment would give negative results.

The temperature of the pond governs the rate at which Ich develops. Sometimes instructions for treatment, give application times, which are based upon aquarium temperatures of 21C. At pond temperatures the cycle is much slower.

In order to successfully eradicate Ich, it is essential to be able to detect two of the life stages, one of which is shown in Fig1. The other is shown in Fig2.

It is difficult to believe that the creature in Fig2 is even distantly related to that shown in Fig1, but it is. Careful use of the microscope is essential to determine the right time for the second treatment of malachite and formalin, which will kill the emerging tomites and early colonies of the parasites on the skin and gills.

The immense increase in size as the parasite develops, gives a fair indication of the nutrients, which it takes from the fish.

.Being so small it is quite difficult to achieve clear pictures as this tiny form of the parasite is in continual motion. This motion is best described as "swimming about" whereas other parasites such as costia move with a definite flickering motion.

Once the tomites are seen on the scrapes, it is the correct time to apply the second treatment of the same malachite and formalin mix. As stated earlier, the temperature of the pond influences the rate at which the parasite develops. An approximate time for the second treatment at 10C would be 14/21days. At 15C - 10/12 days, and at 17C -7/8 days.

It would be a wise precaution to continue to examine any fish showing signs of unusual behavior incase late developing cysts release tomites after the above stated times.

Costia is found on both the skin/fins and in the gills where it can cause serious damage. Like several other parasitic infections it can cause the infected fish to rub their gill plates on the pond floor and sides, together with any other protrusions in the pond. Fish, which are infested with costia often, produce extra mucus and have a grey "bloom" on their skins. However this can be caused by other parasites too so it cannot be taken as a precise diagnosis for costia.

It is a very small parasite but can often be present in such high numbers that the typical flickering motion can be seen, A magnification of at least 100X is needed to detect this parasite: 400X is recommended for positive identification once the very tiny flickering motion is detected. Careful preparation of the slide is essential. Avoid thick mucus preparations and add a little pond water to the slide if necessary such that a thin mucus sample is observed with plenty of "coastlines".

Search along this "coastline" for tiny flickering movements, which are a typical indication of the presence of costia.

Two types of costia are believed to infest fish; one prefers the gills as a site of infection whereas the other prefers the skin. Both are described as "kidney" to "bean" shaped and 10-20microns long, having flagella from 9-18 microns long. (Twenty millionths of a meter long.) (Flagella are whip like hairs, which are used to propel it.)

Both types have simple life cycles in that they divide themselves into two individuals to multiply their numbers: this is known as "binary fission". Costia does not have any intermediate stages but can infect other fish by contact. It can only live for a short while away from a host fish. However, to complicate things a little, costia can hide from treatments by residing in the anus of the fish, only to reappear a short while after treatment. It is not a forgone conclusion that this will happen, but a series of scrape tests is advisable following treatment.
The best recognized treatment for costia is potassium permanganate added to the pond. Potassium Permanganate is a harsh treatment for fish so it is essential that the correct dose be administered.

One and a half grams/ton (1000 litres) i.e. 1.5 ppm, is considered to be a safe effective treatment, which will do a minimum of damage to the biological filters of re circulating pond systems. In order to minimize damage to the filter bacteria (especially that which deals with nitrite) the water should not be circulated through the filters until some of its energy has been expended. If possible re-circulate the filters for first two hours of treatment.

After mixing the chemical with a bucketful of pond water it should be left for approximately half an hour before it is added to the pond. Once mixed in the bucket it will turn the water to a deep purple color. When added to the pond it will make this water purple too. If a pond contains relatively high amounts of organic matter (algae, fish waste etc) the effectiveness of the treatment may be reduced due to the oxidizing effect of the chemical being used up on this matter instead of the costia. A reasonable indication of the effectiveness is the length of time it takes for the pond water to change from purple to pink and then to a brownish tea color. If the pond water becomes "tea colored" in less than four hours, it may be necessary to repeat the treatment 24 hours later or after substantial water changes. Care must be taken not to excessively change the water temperature when this is being done.

Careful use of the microscope will confirm the effectiveness of the treatment.

Click to see microscopic photos of chilodonella   (photos done with microscope and digital camera)
This is another parasite, which can do serious harm to the gills of a fish; it can also cause severe irritation to the skin

This parasite has a typical way of moving which helps identification under the microscope. It tends to stop moving in any particular direction then turns around in a full circle as if pivoted from one end. It is roughly heart shaped and propels itself using cilia that are located at the thinner end of the parasite.

It is believed to multiply by binary fission and is fairly easy to eradicate by the application of malachite and formalin as described for "White spot".

It is one of the few parasites, which are eradicated by salt at 0.5%, and the author has never seen a parasitic problem with koi carp, in re-circulated filter systems where chilodonella alone is involved. On the rare occasions that it has been seen on slides, it has not been present in large numbers. However, it is reported to cause problems in young fish, and may cause problems in ponds with high organic matter.
Click to see microscopic photos  of flukes   (photos done with microscope and digital camera)
Skin flukes are a common parasite of koi and can cause severe skin irritation, which encourages the fish to rub itself aggressively on any protrusion within the pond. This can lead to damaged skin, and bacterial and fungal infections may follow.

They are readily recognized on skin scrapes at a magnification of 30X. 

At 100X the large hooks with which the fluke anchors itself to the fishes skin can be clearly seen. Inside the adult fluke, the young gyrodactylis can usually be seen together with its attachment mechanism, which may be mistaken for another attachment mechanism in the mid section of the adult fluke.
The life cycle of Gyrodactylis is particularly interesting in that each adult parasite carries within it another smaller Dactylogyrus to which it gives birth. This young parasite has yet another embryo within it. The rate at which the parasite reproduces increases with temperature within the normal temperature range.

Being a livebearer, once the adult is killed the cycle is broken.
Gill flukes are also a common parasite of koi carp. They irritate the gill membranes and can cause the fish to aggressively rub its gill covers against any suitable surface in an effort to relieve the irritation. Severe gill damage can be caused if remedial action is not taken.

These flukes are rarely seen on skin scrapes as adults. However the small juvenile stage can sometimes be seen.  Having hatched from the egg this stage becomes free swimming and searches for a new host. If a host fish is not found within a 24hr period, it dies.

Once it finds a host, it crawls along the skin in a similar manner to a caterpillar until it reaches the gills where it attaches to a gill filament using small attachment hooks located at one end of its body. Utilizing the fish’s tissue as food, the young fluke grows quickly to reproductive age and re commences the cycle. Comparing the photographs of the juvenile and adult gill flukes clearly shows the vast growth achieved from the egg to adult stage. As was indicated in the section on "White spot" it is difficult to associate the two stages as being those of the same animal.

The larval and juvenile stage is sometimes eradicated by malachite and formalin but this is not a reliable way of terminating an infection of this parasite.

Gill scrapes can sometimes reveal the presence of these flukes but this is not a reliable method of detection. Identification of the juvenile stage on the skin is reasonably easy. They can be seen as small moving objects at a magnification of 30X. Once suspected, they can then be examined at 100X and are confirmed as gill flukes at this stage by the four "eye spots" at one end of the body. (See Photo)

The adult gill flukes are readily seen in sections of gill removed from a dead fish.( See Photo) Dactylogyrus is clearly differentiated from Gyrodactylis by comparing their heads. The first one has a "four point anterior" and the latter has a" two point anterior", i.e. one looks very much like a paw with four pads, the other like a paw with two pads. It is not uncommon to find both of these fluke species in the gills.

The life cycle of the gill fluke differs from that of a skin fluke in that it lays eggs, which are released into the pond environment where they develop and hatch at a later date. The rate of development depends on temperature and it is believed that eggs can survive for long periods in cooler water temperatures.
An effective treatment is now available which will eradicate both types of flukes at the same time. Used correctly this treatment is considered safe when applied at the precise dose to a pond.
Although this treatment is considered safe, repeated dosing with this chemical remedy can kill koi. It is advisable to establish a good relationship with the supplier of your koi such that advice is sought as to when or if the koi supplied, have been recently treated with this control substance. A minimum of 28 days should be allowed between any treatment received at the koi dealer’s premises and subsequent treatment of the customer’s pond. Recent studies have shown that the active chemicals in Supaverm remain in the fish tissue for a considerable length of time.

Water hardness and temperature play a part in the toxicity of this treatment and although it has been used for several years to control flukes in koi, there is very little documentary advice on its use.

"Supaverm" is added to the pond at the rate of 2.2ml/ton (2.2ml/1000litres i.e. 2.2ppm).

This is left in the pond for three days before any water changes take place. At summer temperatures it is believed that any of the Dactylogyrus eggs, which hatch during the treatment period, are destroyed. As the chemical remains in the fish tissue for some time, it is likely that any gill flukes that hatch and feed on the fish tissue, will succumb to its effects.

CAUTION Goldfish are reported to be killed by this treatment. Orfe and Sturgeon have been seen to survive with no apparent effect.


Click to see photos of trichodina Trichodina is yet another parasite which can cause serious skin and gill damage. It is usually associated with a sudden stress condition. This parasite lives within the pond water where it feeds principally upon bacteria. It is an opportunist parasite, which will infect fish when their immune systems are not functioning at optimum level.

It is readily detected on skin scrapes at a magnification of 30X. (see photo) When alive, this parasite dashes about resembling the proverbial "flying saucer", when dead it resembles a circle with hairs or teeth around its circumference. The cilia, which are positioned around the circumference, are used to propel the parasite.

Like Costia, Trichodina multiplies by binary fission.
The same treatment is used as for Costia. That is Potassium Permanganate added to the pond water at 1.5ppm.

This will eradicate Trichodina from the fish and the water column.

Springtime considerations

Although parasites can appear at any time of the year, the early spring is often when they cause the most concern. Koi, having endured a winter period without full immune system protection, are vulnerable to parasitic and other infections at this time.

A script, which refers to parasites, would be incomplete without an emphatic statement that the appearance of parasites in large numbers in an established pond system, generally indicates that the system is somehow "out of balance".

It should also be stated that disease and parasites are always present, even in the healthiest pond – they are part of the natural order of things.

A severe outbreak of parasites requires treatment. However, in any disease outbreak it is essential to identify the prime cause, which created this state of "un ease". It is important to understand that all treatments, which kill parasites, will at the very least, lightly poison the koi too.

I shudder to think how many koi deaths have occurred because of repeated and unsuccessful treatments for parasites without a thorough and determined search for the prime cause of them, which is usually an environmental factor.

In some ways the parasites of koi are fascinating creatures and observing them through the microscope opens up a whole new world to us. Interesting they may be, but welcome they are not, - in my ponds anyway!

Alan Levington

Notes on Author

Alan began keeping koi twenty years ago when little information was available to koi keepers on how o control parasitic infections.

He has spent much time studying the various parasites of freshwater fishes both in natural lakes and koi ponds. Invariably the environment and overstocking of various fish species has always been the major factor in any disease outbreaks.

He is aware of the limitations of fish keeping in filtered and re circulated systems, but remains enthusiastic that even though optimum conditions may never be reached which compare with a natural balanced pond, better than minimum survival conditions can be achieved.

If you are seeking permission to use bonniesplants.com, logos, service marks, trade dress, slogans, screen shots,
copyrighted designs, photos or other brand features, please contact me
permission requests.
Copyright 2001-2014 - Bonnie's Plants