build a fish pond

You are here: Home > Koi Health Center > Koi Diseases > Costia


Costia fish disease picture

Costia, or Ichthyobodo necator as it is known scientifically, is an external parasite which is active in a vast range of temperatures from 36° to 84° F (2° to 29° C). It is a parasite which can survive both as free swimming organism or attached to a host.

When viewed under a microscope on a skin scrape, it looks like a misshapen circle. However when viewed in its free swimming stage, it simply appears as a small spot moving quite vigorously. At this stage it may be extremely hard to identify because of its very small size, and the fact that it has a short life span of just a matter of hours.

This is a tiny parasite, even in microscopic terms, but its effect on fish when allowed to infest a pond is not so tiny. If left unchecked it can wipe out a whole pond of fish, which is why preventive measures are so important, KoiZyme is the best prevetion method.

It is the first of the parasites that we have seen become immune to the effects of salt. ProForm-C is an effective treatment for Costia.


It cannot be seen with the naked eye so exact identification can only be achieved by taking a skin scrape and viewing under a microscope.

Physical symptoms are similar to other external parasites and these include excess mucus production, which is a common diagnostic sign on Koi. In severe cases reddening of the skin and open wounds may also be present. Often the head will appear as if it has white film over it, and you may also find that the fins of infected Koi may become lifeless and hang in the water while clamping their fins.

If spotted early, costia can early be eradicated, but on young Koi it will develop quickly and can result in losses.

Other symptoms include:

  • Difficulty in breathing
  • Slow erratic gill movements
  • Spend time near airstones and water returns
  • Koi may be seen to rub and flick against the side of the pond.


It becomes a problem when your Koi are stressed. This is normally caused by environmental factors such as poor water quality, changes in temperature, and poor system maintenance. To read more about how your fish can become stressed click here. Installing a heater to avoid temperature fluctuations will help prevent it as well.

It is more likely to appear in colder water, or at times when temperature changes are more likely, for example autumn going into winter and winter going into spring.

A good prevention measure is to use KoiZyme.


Because costia have a short life span when not attached to a host Koi, a one-time treatment will normally be sufficient to get rid of the infection.

Chloramine-T is an excellent treatment for costia.

After treating be sure to also treat any secondary infections which may have occurred as a result from costia.


~Price $40.25 2 lb Jar

Photo by permission of Stars n Sites

You are here: Home > Koi Health Center > Koi Diseases > Costia

ProForm C 1 Gallon is effective in treating Costia

~Price $65.00 1 Gallon treats a 12,800 gallon system three times.

Featured Product


~Price $40.25 2 lb Jar

Enjoy This Site?
Then why not use the button below, to add us to your favorite bookmarking service?

Koi Pond Blog
RSS Feeds

Twitter Button

Costia Beginning

Contact Us

PayPal Acceptance Mark

Return to top
Copyright© 2008-2020.Koi-Pond-Guide.com