Koi fish swimming in a pond with petals floating

As we talked about in an earlier post about quarantining protocols, it is always best practice to quarantine your koi before you introduce it into its new home.

An isolation period allows your new fish time to destress, as well as for any parasites, bacterial, viral or fungal hitchhikers to show themselves. It is easier to treat a smaller tank and fewer fish than a larger koi pond and an entire population.

Looks can be deceiving, and what appears to be a healthy koi might be carrying something that could prove devastating to your pond’s population.

Inspect Your Koi

You can (and should) give all new koi a visual inspection when it is first delivered or brought in. Look for:

  • Discoloration
  • Sunken eyes
  • Raised, loose or missing scales
  • Lesions
  • Growths
  • Fin rot
  • Parasites (some, like adult Argulus, are visible to the naked eye)
  • Rapid or erratic gill movement

Once the koi are in their quarantine facility, watch them as they swim. Frequent and consistent flashing or rubbing can be an indicator of infection or parasite infestation.

Behaviors like jumping, fin clamping, listlessness or sitting on the bottom can be a sign of stress after coming into a new environment, but won’t last long once they have acclimatized. If they continue after a few days, it could represent a more serious issue.

There Will Be Bugs

We have said it before and we will say it again—if you have a koi pond, you have bugs. Some of them are instrumental in maintaining balance in the flora and fauna in your pond. Aeromonas, as much of a threat as it can pose in the spring, helps to break down fish waste. In other words, bugs HAVE to be there for naturally occurring processes to function.

Aeromonas hydrophilia under an electron microscope
Aeromonas hydrophilia

Some of these bugs, however, are opportunistic pathogens. Given favorable conditions, these bugs will replicate—and quickly, too! And the favorable conditions they require are never good news for your koi.

Water quality is always the alpha and omega in koi keeping. Cleaner water (which DOES NOT always translate to clear water) controls the conditions in which many of the bugs live and thrive.

Stressors for your koi, such as low dissolved oxygen, excess waste and/or high levels of dissolved organic carbon, make koi more susceptible to bugs that thrive in the same conditions.

When you are checking your water parameters, make sure that you check all of these:

  • Ammonia—should be ZERO
  • Nitrites—should also be ZERO and anything over 0.25ppm becomes dangerous
  • Nitrates—not as lethal as nitrites,  so levels up to 60ppm are still considered safe
  • Dissolved oxygen—5mg/L is the minimum necessary for a healthy population
  • pH—7.0 is considered best but 6.8-7.2 are still within the optimal range
  • KH—50-150ppm is a good range to help stabilize the pond’s pH

Filter Cleaning

Removing debris and waste from a pond is vital in maintaining good water conditions. There are numerous types, styles, and preferences for filters to help keep healthy water parameters and conditions.

The recommendation for the frequency of koi pond filter cleaning is four times a year. It needs to be done sparingly, as cleaning it too well will remove the beneficial bacteria that has grown on it, which in turn means that you will have to recycle your pond.

Remember—brown does not mean bad! Once your colony of beneficial bacteria has taken root they will take on a brown color.

You will need to remove thick green or black sludge from the inside of your filter, as it is stopping oxygen from reaching the bacteria that need it to function.

No matter which style of mechanical and biological filter you opt for, you should clean it periodically.


Do not use tap water to clean your filter media. The chlorine will kill the beneficial bacteria. Use pond water instead.

Preventative Quarantine Dosing

Debate rages about using pond-wide measures in the spring to control bugs (especially parasites and bacteria) before they become an issue. It falls on the experience, successes, and failures of the practice itself.

As with any treatment that you administer to get rid of bugs in your pond and on your koi, you have to be mindful of overuse. Even when correctly dosed, your koi can eventually develop a resistance to that drug. This, in turn, will reduce the efficacy of future treatments in the same pond, making it more difficult to control or eradicate the bug.

In a smaller environment, like a quarantine tank, some view it as something akin to an inoculation. The bug count, with a little help from some chemicals, stays low enough so that the new koi get a chance to build their defenses to those bugs.


Temperature is an important part of the quarantine process. Koi are poikilothermic, meaning that their internal temperature is determined by the ambient temperature of their environment.  This means that the strength of a koi’s immune system is directly tied to the water temperature of its environment.

Koi immune systems function best in temperatures above 45ºF. In general, within good water parameters, the warmer the water, the more effective the response to any bug will be.

Also, existing bugs begin to manifest themselves at specific water temperatures.

  • Aeromonas and Pseudomonas are in all pond water but are more likely to do damage at lower temperatures. ”Aeromonas Alley” is between 40-60ºF when the koi’s immune system is still getting back up to speed.
  • Parasites like Costia or Trichodina are most likely to proliferate at temperatures below 65ºF.
  • The signs of Koi Herpesvirus (KHV) become evident at temperatures between 68-80ºF.

The recommended temperature for a quarantine period is in the lower- to mid-70s. It is important to remember that at higher temperatures there is less dissolved oxygen, too.

Types of Medications to Use

Medications can be used to help with a laundry list of bugs that can affect your koi’s well being, both in a quarantine and pond-wide situation.

These are some of the more prevalent treatments used to combat bugs. Some can be used in conjunction with one another while others should not.

As with any treatment for your koi, you should always know the correct dose and length of treatment that corresponds to the size of your pond or tank.

Too little and it will be ineffective. Too much and it can be fatal.

You should also know your contraindications, as well as what an effective treatment looks like as it progresses (or doesn’t).


Pond salt for Preventative care in Koi Ponds Somehow this simple seasoning spices up the conversation when it comes to treating a sick fish. Koi keepers have used salt to calm stressed fish in new environments for many years.

Salt is an inexpensive means of parasite and bacteria control. Higher salinity makes an environment inhospitable and so helps to reduce the count of both parasites and bacteria in the water.

Salt is an irritant. In response to that irritation, the koi produces more mucus. This forms a protective (and antimicrobial) barrier to bugs intent on finding a foothold on the koi.

It simultaneously dries up any excess mucus on the gills, and so helps to control bacteria or parasites that make that extra mucus home.

Additionally, higher salt content helps with the osmoregulation function of the gills. The koi will not have to work as hard to get water back out of its body, which is helpful for a stressed koi or one with excess mucus on the gills.

Salt is not a cure-all. It should not be looked to as one, nor as a long-term solution. However, it can be an effective first response. It can then be followed with further chemical warfare against a number of different bugs.


Particulate matter and dissolved organics can negatively impact many of the chemicals used to treat bugs.

Water clear of detritus, submerged organics, and other waste is the best way to help with a successful treatment.


This is one of the preferred forms of parasite control, both internal and external, amongst koi keepers. It is effective against intestinal worms, as well as skin and gill flukes.

Praziquantel is gentle on koi and filters, and it doesn’t require water changes to dilute it.

This form of treatment is still considered an expensive solution though, especially if the pond or quarantine tank has a high volume.

Praziquantel can be mixed with Formalin and Malachite Green in a later treatment, which will help to limit almost all the major parasites that affect koi.

Potassium Permanganate

This caustic alkali is used as a one-shop-stop to address fungus, bacteria and protozoan parasites (like Trichodina, Costia, and Chilodonella) and flukes, killing the parasites through oxidation of the parasite’s cell wall.

It can limit the number of bacteria like Aeromonas, too. However, potassium permanganate can also destroy beneficial nitrifying bacteria.

Use extra caution when treating with potassium permanganate.  It is a poison, and incorrectly dosing your pond and tank can quickly kill your fish.

Malachite Green and Formalin

This anti-parasite mixture has been used for many years by koi keepers. These two treatments are most effective when used together as a mixture and are used to control skin and gill flukes, Costia, Trichodina, Chilodonella, and Ich. They can also be used to treat bacteria and fungus.

MGF is used to combat some of the parasites with longer life cycles (especially Ich), as it has a long active life in solution.

On its own, Formalin is primarily used to treat simple (single-celled protozoans) and complex (multi-celled metazoans) parasites.

Malachite Green is useful in treating fungal infections that are often secondary once bacteria or parasites have done their damage.


These two treatments are used to control anchor worms and fish lice. They work by disrupting the reproductive process and life cycle of the parasites.

ProForm is less expensive and less harmful to any pond dwellers other than the koi that you might have in your pond or tank.

Neither are harmful to your pond’s biological filtration system.

Chloramine T (n-chloro-para-toluene sulfonamide sodium salt)

Chloramine T is a mild sanitizer that has biocidal qualities. It is a good complement to a salt regimen when used in a quarantine tank.

This treatment is used as a control and cure for bacterial gill infection. Chloramine T is also used to treat visible bacterial infection of a fish, as well as any that are prevalent in the water.

The hardness of your water can influence the amount of Chloramine T you add to your pond water. The harder the water the higher the dose will be, especially at lower pH values.


Antibiotics should NOT be used as a preventative. Bugs are incredibly resourceful. After millions of years of successfully adapting to changing environments, they are keen survivalists. Adding antibiotics to pretreat can cause bugs to develop resistance. This then makes getting rid of them much more difficult.

If a disease or ailment should arise that requires antibiotic treatments (and is validated through scoping) then it is time to look at means of stopping the spread of the disease on the koi.


Enrofloxacine (Baytril) is the most prominently used antibiotic treatment amongst koi kichi for ulcers.

It is an injectable, broad-spectrum drug that is most effective against gram-negative bacteria (like Pseudomonas and Aeromonas). Baytril works by enzyme inhibition that stops the bacteria from replicating.


Tricide Neo Antibiotic dip for koi fishThis is another popular antibiotic. Used as a dip, it is an effective treatment for topical bacterial infections (like ulcer disease, fin rot, tail rot, and mouth rot).

It works by lysing the bacterial cell wall and then allowing the active antibiotic neomycin into the cell which then eliminates it. It is effective against both gram-positive and gram-negative bacteria.

Tricide-Neo will damage your beneficial bacteria, so it cannot be released into the quarantine tank or the pond. Once the fish has been dipped, it will have to be rinsed off before being released back into the pond

As intense as this type of maintenance and koi care might seem, it is an effective way of giving your new koi, your old koi and your pond the best shot at staying healthy in the long haul.

2 responses

  1. Julie Plott :

    Fantastic article I’m going to hang on to it to remember the treatments, I’ve heard they are going to stop selling meds to individuals , that I need to purchase them now, when does it go into effect? Thank you!

    1. Josh LeFever :

      Hey Julie, the medications mentioned in the article are not scheduled to become illegal for consumers to purchase, so there is no need to stock up on them.

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