A variety of fish illnesses strike all pond fish, and some are specific to only carp, such as koi. This listing doesn’t include all possible parasites, viruses, and bacteria types, but it does list some of the most common, as well as some that are less common but are particularly unpleasant. Some of the problems listed below are relatively easy to cure, others are curable with a great deal of effort, and still others don’t have a cure.


Anchor Worm

This crustacean parasite is one of the more common parasites found on pond fish, and it can be seen by the naked eye unlike many other parasites that require a microscope. This parasite begins in the gills, but moves to the body as it matures, digging in like a tick with its head, under a scale on the fish. Anchor worm eggs are laid in the pond and will remain dormant until environmental conditions are correct for them to hatch, and the juvenile parasite then goes out looking for a host.

Removing an anchor worm is similar to removing a tick, using a pair of tweezers. If you have a large koi, you may need to find a specialist to anesthetize the fish before you can remove the anchor worm. If you have a small koi, the process will include:

  1. Capture the koi, gently holding it down so you don’t damage it
  2. Dab the parasite with a piece of gauze or cotton ball soaked in potassium permanganate to get the parasite to release its hold on the fish
  3. Grab it with the tweezers and remove it

After an anchor worm is removed, there is a wound left behind that will be susceptible to bacterial infection setting in. It is therefore recommended to quarantine the koi to a smaller pond to treat it with an antibacterial medication as it heals. If a small pond is not available, an appropriately sized and filtered aquarium can be used while the fish heals.


Costia is not very common, but it is found from time to time. This protozoan is not one viewable without a microscope, but when it attacks a koi, it can be found in the gills and on the scales. Koi with a costia infestation will scratch against objects, have a pale color that looks cloudy, and often have clamped fins. Often, this parasite isn’t the primary problem and is instead a secondary problem that occurs after the fish is already in some other sort of distress. Treatment is usually with copper or acriflavine, but both of those medications have their own possible side effects so medicate very carefully. Also, be prepared to do water changes wherever the fish are being medicated with these chemicals. Some success has been reported with strong salt baths as well.

Fish Lice

Fish lice, when found on a fish, are a cause for serious concern. Typically, fish lice are found in the gills of a fish when present. This parasite is very difficult to treat, as most anti-parasitic medications don’t work. Speaking to an aquatic vet may be required to obtain the proper chemical medication for fish lice, as it is not generally available to the public. There are new products on the market, such as Microbe-Lift Lice and Anchor Worm that may prove effective against fish lice. These medications contain a chemical known to work. Fish lice attach to a fish with their sucker-like mouths, and they inject a toxic substance into the area around. This causes a great deal of irritation to the fish, which will cause it to scratch against things as well as jump and act in an erratic manner.


Flukes on the skin and gills of a fish are somewhat normal and are nothing to worry about, unless they become an infestation that gets out of control. Flukes are not visible to the naked eye, and a microscope would be required to see them. A fish infested with flukes shows the same symptoms as with most other parasites, such as scratching themselves against things in the pond, jumping out of the water, and general signs of irritation. Treatment of this parasite is fairly straightforward, with products containing formalin and malachite green being the most commonly used treatments.


Ich is a very common protozoan infestation that can spread rapidly through a pond, infesting every fish it can. Symptoms are small white spots the size of salt crystals or a little larger, loss of appetite, scratching against things, and lack of activity. The fish will die from ich itself, or from starvation if this is not treated right away. There are a variety of ich medications available on the market today that are effective.


This irritating protozoan parasite is another that irritates the skin of the fish, causing discomfort, scratching, and erratic behavior in the fish. This parasite is not visible to the naked eye; it can only be seen under a microscope. When viewed under a microscope, it is almost perfectly round with very fine cilia around the edge. Anti-parasite treatsments include chemicals such as formalin, malachite green, potassium permanganate, and salt.



This gram-negative bacterial infection is often mistaken for fungus because its visible symptoms look very much like white tufts of wool on the mouth and body of the fish. This disease is very contagious from fish to fish and will easily spread in a pond. If it is only noticed on one fish, try to quarantine the fish and medicate it while keeping an eye on the rest of the fish in the pond. If another fish shows symptoms, then it is best to treat the entire pond. This disease will usually only show itself in fish that are already stressed due to recently being moved, another disease already affecting it, bad water quality in the pond, or other environmental conditions.

There are many medications on the market today that will treat bacterial infections. Something along the lines of Melafix is a good place to start.


Dropsy is sometimes referred to as “pine cone disease” because of what happens to the shape and scales of a fish that suffers from it. The fish will bulge out in the middle, and its scales will stand out, giving it the appearance of a pine cone. There are several possible causes for dropsy in fish. Bacterial infection, tuberculosis, and tumors are all possible reasons for dropsy. The bacterial infection is treatable with antibacterial medication, while the tumors and tuberculosis are not treatable. So, the first place to start with dropsy is to quarantine the fish if possible, and begin treatment with antibacterial medications.

Fin Rot

Fin rot is caused by any one of several different types of bacteria, and its effects are usually very obvious. The fins or tail of the fish will begin to break down and have the appearance of rotting away. Once fin rot has taken hold, it is easy for secondary infections and fungus to show up on the fish, due to it being under such stress. Isolating the fish and treating it with antibacterial medication will usually stop the rotting process, and, over time, the fins and tail of a fish will usually grow back and be just fine.


Koi Herpesvirus, KHV

A virus that is very contagious among koi and carp is koi herpesvirus, or KHV. The first reported case of this virus was in 1998. KHV stays with a fish for the duration of its life, so if it survives initial exposure, the fish will always be a carrier of the virus. When initially infected with KHV, a koi will often die within the first 48 hours of exposure. Some of the common symptoms of KHV are sunken eyes, bleeding gills, pale color patches on the body, and blisters and sores on the body.

Carp Pox

Carp pox is another herpes virus in koi and goldfish, which is neither curable nor dangerous. Once fish have it, they will carry it forever, but it may not always be visible. The visible indicator of the virus will be waxy wart-like growths on the fish, which appear when the water is cold. Usually, when water warms up, the pox will hide and will not be visible at all, but will generally reappear as soon as the pond water starts to cool down in the fall. Do not try to remove the growth from the fish—it will only cause open wounds that will then possibly become infected. The virus is unattractive, but is not fatal. Most carp and goldfish have developed a resistance to it, so having one fish in a pond of 50 or 100 does not mean that the others will ever develop it.



This fungus or mold is very commonly seen in koi, but is usually a secondary infection that takes place after an injury. Saprolegnia feeds on dead tissue, so it most likely appears right on top of a sore on the fish. But once it has taken hold, the fungus can spread out past the dead tissue and start to destroy live tissue. If not treated, this fungus will often be fatal to the fish as it is slowly eaten away. A variety of antifungal medications on the market will take care of this disease. The effectiveness of the medication will depend on how quickly you treat the fish.

While this is not a complete list of diseases that could affect your pond fish, hopefully it will help determine what your carp may have, and, sometimes just as important, what they do not have.

About The Author Pet Expert

comments (0)

Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>