H & I: Pond keepers A to Z … Hormones To Isolation Tank


From the Latin word for ‘arouse’. Hormones are regulatory substances that are released by specialised cells from all over the body, causing an effect on distant ‘target’ cells or organs.

Hormones are incredible biological chemicals that are transported throughout the body in the blood to produce quite amazing responses. Hormones are secreted by fish in response to changes in their environment.

Rapid changes such as being netted stimulate the release of adrenaline which enables fish to mobilise quantities of oxygen and energy to fuel a rapid escape. It is actually the side effects of hormones released by fish to counteract stress that unfortunately makes them susceptible to ill health. Hormones are also used extensively in the intensive culture of koi where mature broodfish are injected with specific sex hormones at prescribed time intervals to stimulate the release of eggs and sperm. The koi farmer’s understanding and experience with hormones has allowed the effective manipulation of broodfish enabling predictable spawning results.

Haemorrhagic Septicaemia

Septicaemia, commonly referred to as ‘blood poisoning’ is largely a symptom of a severe bacterial infection. Bacteria and their toxic by-products can build up in the blood causing further complications, such as haemorrhagic septicaemia.

Commonly associated with extensive bacterial infections such as those encountered by koi suffering from ulcers or other extensive lesions. If left untreated haemorrhagic septicaemia will cause the death of a koi. Symptoms are largely internal and are only really evident after dissection. However, in extreme cases, the accumulation of blood at the base of the fins and around the eyes and vent may be evident. If caught early enough, haemorrhagic septicaemia can be treated using antibiotics (injection) that are effective against the specific pathogenic bacteria. Through the spread of resistant strains of bacteria, effective treatment is becoming increasingly difficult.

Hi (Pronounced ‘he’)

One of the more commonly used descriptions of colour in koi, describing red areas of colouration. Its wide use is attributable to the description of red in the popular kohaku, sanke and showa varieties. It is also commonly found as a prefix describing a red form of a variety e.g. Hi utsuri or Hi showa where red is found in abundance.

The depth and clarity of Hi on a fish can be a useful guide to the quality and lineage of a koi. The deeper the red, the better the quality and higher value of the fish.


Good health may be defined as when a koi’s body is functioning properly, displaying good colour, behaviour and vitality. Achieving good health in pondfish is the result of a host of factors interacting with the fish’s body. Stable water quality and a complete and balanced diet are the two main factors contributing to health. Other environmental factors affecting health include husbandry and handling. As fish live in an extremely dynamic environment, the balance between health and stress can be a fine one where a decline in a single environmental factor can cause a downward spiral in health. As koi keepers, we have complete control over the factors that determine the health status of a fish (besides genetic influences) and a pond full of healthy fish is a testimony to the pond keeper’s long-term efforts.

Handling Koi

Handling koi can be compared to playing snooker or bricklaying in that it is not as easy as it first appears.

Smaller koi are easily caught and handled using a hand net, where the fish will comfortably be held within the confines of the net. Larger fish are more effectively moved from net to bowl or from bowl/floating basket by physically handling the fish. The term handling seems to suggest quite a clumsy procedure and should perhaps more accurately be called ‘cradling’.

Approaching a fish in a confined basket, net or bowl, with its head pointing towards you, the koi should be swiftly removed from the water by placing the fingers of the left hand under the vent area while resting the koi’s head on the open palm and wrist of the right hand, moving either hand in time with any side-to-side movement of the koi.

Practice improves with technique and confidence, the latter making all of the difference. If at all possible, avoid moving koi by hand between areas where there is no water to catch a fish should it really be determined to wriggle free. And if a koi does decide to wriggle, don’t panic or try to grip the fish harder, but let it return to the water as gently as possible to try again.

Koi handling should be kept to a minimum and is only necessary when buying, treating and showing fish. It is also carried out when breeding koi, but in these instances the koi are sedated for most of the time.


Unfortunately, most fishkeepers are familiar with whitespot, known as Ichthyophthirius multifiliis (or ich for short – thank goodness!)

It is very easily diagnosed and if spotted and treated early enough is also very easily controlled. However, it is notorious for spreading rapidly through fish populations and infecting fish to such a degree that they may not recover. The pinhead white spots are the encysted stage of this protozoan parasite which feed on the koi’s body fluids and cells. Once mature and free from the koi, the cysts rupture to release hundreds of infective parasites. It is these ‘swarmers’ that are controlled by pond treatments with the encysted forms being resistant to any medication.

Infected fish are irritated by the parasites and will scratch and later gasp as their gills become infected. Whitespot is present in every pond, waiting for a stressed fish to infect. A common stressor leading to an outbreak of whitespot is a sudden drop in water temperature, perhaps as a result of a water change.

Internal Parasites

Although other pets such as cats and dogs are regularly treated for internal parasites such as tapeworms and roundworms, internal parasites are rarely acknowledged as a significant problem in koi. Sometimes with quite complex lifecycles, the effects of internal parasites on koi are more chronic and long-term than other parasites. Their occurrence in fish is not associated with those fish becoming susceptible through stress but is a factor of their environment and the disease status of other fish in the pond.

Similarly, it is difficult to diagnose an internal parasite problem as unlike other more familiar koi parasites, there are no clear or visible symptoms. If koi growth is stunted or fish appear to have swings in their appetite, there may be other causes of the problem besides internal parasites.

Even if such a situation is suspected, treating against these parasites can pose a real problem. Most, if not all useful anthelmintic (worming) treatments are available by prescription and are administered orally. We know how difficult it can be to get a cat or dog to take a worming tablet never mind a koi!


Iridocytes are a type of colour cell found in koi and other fish that give them a silvery glint. There are two types of colour cells found in koi:

Xanthocytes which contain chromatophores. These can be found in as many colours as koi exhibit and can generally be enhanced by feeding colour enhancing diets.

Iridocytes. These cannot be improved by feeding a colour enhancing diet and are responsible for giving fish the silvery iridescent reflection to their scales. When we see fish scratch or scrape and their flanks catch the light, the glinting is caused by the iridocytes.

Immune System

A koi’s immune system is a combination of defence mechanisms that enable koi to resist pathogenic micro-organisms such as bacteria, fungi, parasites and other foreign bodies. The first line of defence is a koi’s mucus layer covering its delicate skin. The scales also offer a physical barrier against damage and infection. There are sites that are more prone to infection than others such as the delicate membranes of the gills and intestine. If a pathogen successfully breaks through the initial line of defence, a koi will defend itself through the action of antibodies and specialised blood cells, attacking any foreign bodies in the bloodstream.

A koi keeper will have experience of the koi’s immune system during the in between months of spring and autumn. As koi are coming out of their winter break as water temperatures approach 10 degrees C, a koi’s immune system is slow to respond to the rise in temperature. Meanwhile, many species of pathogenic bacteria and protozoa multiply and become quite active, threatening to infect a suppressed fish. The same is true for koi approaching autumn, but the risks are not as great. It is quite common practice for ponds to be dosed with a broad-spectrum antibacterial treatment during these two periods to reduce the risk of infection of suppressed koi.

Insect Larvae

Insect larvae may either be considered as a pest or a tasty treat depending on the size of the koi involved. The larvae of dragonflies and great diving beetles are carnivorous and will even give a juicy finger a painful nip. Likewise, they will readily take koi fry with larger larvae, even attacking koi up to 2″. Insect larvae are very rarely a problem in traditional steep-sided koi ponds but are more likely to be a factor in ‘natural’ or well planted garden ponds. They are a particular hazard to fry in koi ponds, where pond management techniques can reduce their impact on fry numbers.

Isolation Tank

A handy standby piece of equipment for the more serious pondkeeper. Installed with its own filter system which is best left running, an isolation tank or pond is ready to take individual fish that require dedicated care. It should be ready for quarantining new purchases or isolating sick fish which may need an intensive treatment regime.

Care must be taken to ensure that an isolation tank increases the chances of a sick fish surviving or that while a fish is being held in quarantine that its health is not compromised by second-rate water quality. To this end, an isolation tank must offer sufficient space, excellent stable water quality and easy access in an otherwise quiet environment. If this is not possible then it must bring into question whether anything is to be gained by using an isolation tank that does not offer first class conditions and that koi are best left and treated in situ.

Kill blanketweed and string algae.