Parasites and fungus common problems facing koi keepers

Koi parasites and fungus

These are two problems that regularly affect koi health, Both can be dealt with fairly simply when they are at an early stage. Fungus, if allowed to spread, will quickly overwhelm fish. Large numbers of parasites can cause physical damage to both skin and gills and are often the precursor to bacterial disease. Diagnosis and treatment is often a chicken and egg situation.

Disease diagnosis

Diagnosis is really ‘detective work’. The clues are usually there for the careful observer and if you look carefully enough. If you are methodical and try to eliminate as many possibilities as you can, without jumping to conclusions, then you may well be led to the correct conclusion. At the very least, you will have gathered important observations that will assist someone else, more expert than you, in reaching a diagnosis or coming to a conclusion. There are two main problems. The first is that there are many clinical signs that can have more than one cause. For example, ‘flashing’ and leaping out of the water could be caused by irritation from parasites, but it may just as well indicate a water quality problem. For this reason it is important to carry out regular water testing and to keep a record of readings in order to assist in arriving at the correct diagnosis.

The other difficulty is deciding whether the apparent cause of poor health is the primary or secondary factor. For example, a fish that is unwell is more prone to parasites, which may begin to multiply and become a health problem in their own right. But this would be a secondary  infection brought about by the original problem. This is what I meant when I compared making a diagnosis with being a detective. It is often very easy to jump to conclusions and thereby overlook the initial cause.

However, bearing in mind these potential complications, we can, for the sake of simplicity, divide health problems into three general categories, namely:

  • external or skin problems: fungus, parasites, bacteria
  • gill problems
  • other problems


The most common fungi to affect fish is Saprolegnia. Often it is first noticed as white or grey cotton wool-like tufts sticking out of the skin or fins of fish. It is sometimes tinged green with trapped algae. Fungus is nearly always a secondary problem, meaning it has occurred because of some other initial damage. The primary cause may be bacterial, parasitic, water quality or the fish may have physically damaged it’s skin or fins in some way. It can be a serious problem because the fungus will spread and invade healthy tissue. Treatment with a proprietary anti-fungus compound will normally clear the problem, if applied early enough. but it is just as important to treat the underlying primary cause to prevent the fungus reappearing.


Fish attract a whole range of microscopic parasites, some of which live on the outside of the body on the skin, fins and gills, while others invade the internal body tissues and organs. Those living on the outside are known collectively as ectoparasites, while internal parasites are called endoparasites.  Although some endoparasites can be quite serious, it is fair to say that most parasitic problems in ponds involve ectoparasites. The latter vary in size and structure, from minute single- celled protozoa, such as Costia and Trichodina, to worm-like skin and gill flukes which may be up to 1 mm long (see the main health section for more details).

In small numbers they are usually no problem but, given their enormous reproductive capabilities and the right conditions, they can rapidly multiply and overwhelm the fish. In severe infestations fish can become literally smothered. Large numbers can endanger the health of fish because of the damage they do to delicate skin and gill surfaces. For instance. flukes and leeches have attachment devices – normally suckers and/or hooks – which enable them to stay attached to the fish and move freely over the body. The damaged tissues then provide ideal entry sites for secondary fungal or bacterial infections.

As you can probably imagine. the presence of large numbers of parasites is extremely irritating and one of the main signs of a ectoparasite infestation is the sight of fish rubbing themselves against a solid object, often bottom drains, in an attempt to ease this irritation. This rubbing action, during which the fish often turn on their sides, is called flashing. Other signs of irritation are fish leaping out of the water and swimming at speed around the pond, apparently in great irritation.

The other response is an increase in mucus production, often leading to ‘slime disease’, whereby the mucus forms a grey/white layer over parts of the body. If fish lie motionless in the pond or gasp at the water surface, or hang around water returns, this may also indicate a parasitic problem, but it is also indicative of several other serious problems, and the beginner would be well advised to seek more expert help in this situation.

Although the above signs of irritation are useful indicators of an ectoparasite disease, it should be remembered that poor water quality, especially high ammonia or nitrite levels, will cause the same response or ‘clinical signs’. In reality, the only certain way to make a diagnosis by carrying out a microscopic examination of a skin scrape, which has the advantage of determining the actual species involved and the severity of the infection.

Other specific problems worthy of mention are ‘white spot’, which is fairly easily cured but can be fatal if not rectified quickly. This is easy to identify, as the fish looks as though it has been sprinkled with fine grains of salt. Small, white cysts up to 1 mm in diameter are clearly visible over the body and fins of the fish.

When the trophonts inside these cysts mature, the cysts rupture, releasing the mature trophonts into the water. The damage done as the cyst ruptures can provide a site for secondary infection. Meanwhile, the mature white spot trophont reproduces, releasing hundreds of ‘swarmers’ or tomites into the water to re-infect the fish. Larger ectoparasites, such as Argulus (fish louse) and Lernaea (anchor worm), are visible without magnification.

These are two real nasties which should be treated as soon as possible. Argulus is up to 10 mm across and is often overlooked because at a glance it appears to be just a small black mark. However, if your fish are ‘examined’ regularly these lice should be easily spotted and if watched for a few moments these black’ spots will be seen to move. If Argulus is suspected, a close examination in a shallow bowl will quickly settle any doubts. The first sign of Lernaea is what appears to be fleshy pieces of cotton, up to 20 mm long, attached to the skin or fins of the fish. Closer examination shows the body of the anchor worm, with its head buried into the flesh of the fish, with a trailing body and two egg sacs at its tail end.

With the exception of a few species, such as white spot, Argulus and Lernaea, most ectoparasites are not harmful in small numbers, being similar to fleas on a cat or dog. The major danger comes when, for some reason, the number gets out of control. Without the aid of a microscope it is difficult to determine just how severe the infestation is. However, it would be a mistake to treat the pond with an anti-parasite treatment every time a fish rubbed against the side or jumped out of the water. Equally, it would be wrong to ignore things if the fish were obviously suffering from great irritation and were constantly leaping around or tearing around the pond, rubbing along the bottom and sides.