D: Pond keepers A to Z … Depressing to Ectoparasite

D is for Depressing!

The majority of the D’s seem to be negative – disease, dropsy, Dactylogyrus, diet(my wife thinks so!) and I could have included death!

Dactylogyrus (Gill Fluke)

One of the many parasites of koi that will cause them to flick and scratch through irritation. The gills are very appetising for parasites as they offer a constant source of food that is very accessible, with the gills having such a thin penetrable membrane separating the nutritious blood from water. However, there is also a strong stream of water pumped across the gills which means that to hang on, gill flukes need a serious pair of hooks to hold on – hence the irritation to the fish.

A case of Dactylogyrus can be difficult to diagnose in koi as they are barely visible to the naked eye and are tucked away out of sight in the opercular chamber. It is not possible to take a live sample as there is a real risk of damaging the delicate gill tissue. However, it is rare for a gill fluke problem to be an isolated infestation and usually occurs with an increase in other ectoparasites such as Chilodinella, Trichodina and Gyrodactylus which can be easily diagnosed with a skin scrape. Thankfully, the treatment for the skin fluke will also be effective against the gill fluke.

Treatment can be effective using a series of prolonged salt baths with an investigation into the cause of the initial problem. Another extremely effective yet controversial form of treatment is using an organophosphate compound containing trichlorphon. This product although effective is not licensed for use in aquaculture (See Dipterex later).


A health disorder, commonly brought about by subjecting koi to stress. The way fish respond to stress brings out some extreme physiological pressures which leave them susceptible to attack from a range of disease organisms. Viruses, bacteria, fungi and parasites which cause disease are highly opportunistic organisms often present in low numbers just waiting for a weakened fish on which to thrive and multiply. This will lead to the fish exhibiting the typical symptoms associated with that disease.

There is generally a traceable cause for a disease incident, usually being linked to a water quality, handling or a cross-infection event. Either way, most diseases if diagnosed early enough are quite treatable with plenty of advice and medication available for the accurate diagnosis and effective treatment of disease.


A scale variety in koi reputed to have been introduced from Europe, most frequently found in Shusui and Metallic varieties. Classic Doitsu scale patterns show enlarged mirror-type scales arranged along either side of the dorsal fin along the back as well in some fish along the lateral line.

Because the majority of skin on a Doitsu koi is scale-less, pattern on Doitsu fish generally show better definition. In addition, from experience, when injecting broodstock with hormone to induce spawning activity, Doitsu fish are a lot easier to work with as there is no need to locate the needle beneath a scale for injection.

Did You Know?

When breeding Doitsu fish, during a Doitsu cross, 25% of the fertilised eggs will contain a lethal gene which prevents those affected from developing beyond the embryo stage. So do not worry if you find large quantities of larvae on the bottom of a hatchery tank 2-3 days after hatching as their death is through a quirky genetic trait and is entirely natural (but strange!)


It is far easier to describe the symptoms of dropsy than to identify the specific cause of it. Fish suffering from dropsy exhibit the classic pine-cone appearance where the scales become raised and protrude from the smooth body surface and the eyes may even protrude from the head. In extreme cases, the scales may become reddened through localised haemorrhaging when fluid accumulates within the body cavity.

There is no apparent single cause of dropsy and it usually only affects individual fish. As there are uncertain causes of dropsy, precise treatment can be difficult and affected fish are best isolated to reduce the likelihood of any infection to other fish. It is rare for dropsied fish to respond to medication.


A very important consideration when designing a koi pond is depth. Recognising that it is far easier to design a deep pond than dig it out, a depth of 3 feet should be regarded as the absolute minimum for a koi pond.

Deep ponds offer several advantages, namely stability in water quality by packing more volume into a given area and by virtue of the fact that deeper ponds maintain a more stable temperature with more water being insulated from the cooling surface. The bad news about any deep pond is that it needs to be excavated and fish can be difficult to net with lengthy attempts of trying to catch koi with long and unwieldy nets potentially stressing other pond inhabitants.


A koi’s diet is instrumental in determining it’s growth rate, health and colouration. Although completely unrecognisable from a koi’s natural and wild diet, manufactured koi and pond foods are highly researched, complete and balanced diets that are formulated with the koi keeper, koi and water quality all in mind. Koi diets are based on successful carp food formulations where the protein content is provided from both vegetable and animal origin. It is a low fat diet where the koi gains most of its energy from carbohydrate. In fact, there is little difference in analysis between a koi’s diet and one that would be suitable for us to eat – except perhaps for the taste!


This is a series of physical and chemical processes that breakdown food material from insoluble, complex compounds into less complex and soluble products. Koi digestion is quite incredible in that the only teeth they have are in their throat and they don’t have a stomach or acid digestion. Their gut is incredibly simple, being similar in structure along its entire length and what a length….a koi’s gut is usually twice the length of it’s body.

Dorsal Fin

Not a very noticeable feature when koi are viewed in a pond from above. However, a koi’s dorsal fin can be used as an indicator for its health. Easier to notice when viewed in an aquarium, an erect dorsal fin indicates that the koi is healthy and in good condition while a dorsal that is clamped or folded close to the body is likely to indicate an unhealthy stressed fish. It is a sign that you should investigate water quality.


This is the trade name of an organophosphorous compound manufactured by Bayer and used against insect pests of vegetable and root crops. It is not licensed for use in aquaculture or koi keeping but its active ingredient, trichlorphon is excellent at treating larger ectoparasites such as anchor worm, fish lice, skin and gill flukes. Trichlorphon used to also be available as a Japanese antiparasitic treatment called Masoten but its use has been curtailed recently, When handling organophosphorous compounds, great care should be taken as they are essentially a group of nerve toxins. Severe overdosing problems can occur (kinked backs in orfe and other fish) and pond water should not be touched for several days after dosing. Trichlorphon belongs to the same family of chemicals as those used in sheep dip – which have been gaining some very negative press recently for their effects on farmers, so take care.

Jargon Buster:

Ectoparasite. A parasite that lives on the outside of a koi’s body. An endoparasite lives internally.

Kill blanketweed and string algae.