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last updated 29th December 2021


Parasite treatments for Koi carp

Having taken a skin scrape from the chosen Koi, examined this carefully under a microscope, and identified the particular parasite causing the issue, learn how to choose and administer the correct medication to ensure success - every time.


There is a huge range of parasite treatments available to the Koi keeper, many commercially available from a range of manufacturers. and all of course claiming to eradicate all known Koi parasites with ease - if only it were that simple!


The first thing to remember is that most commercially pre-bottled medications contain one or more of the generic medications that we list later on this page as being the de-facto standards in Koi medications. Nothing wrong with that, medications pre-prepared for us in that way help decide on the best one for our purpose, and make it easier to calculate and administer the correct doseages - in theory at least. The problem with this approach is that doses suggested with nearly all pre-bottled medications can be widly out for your particular water chemistry. The doseages suggested will inevitably be at the low end of the scale simply because the manufacturer cannot risk prescribing a dose that might damage or even kill your beloved Koi. So they have to, logically always err on the side of caution, which unfortunately means that in roughly 2/3rds of cases the dose recommended will be too weak and simply wont work.


As we allude to above, the single biggest problem with all parasite treatments is getting the dose right, too weak and the treatments simply wont work - too strong and you risk damaging or even killing the Koi. So the first thing is to calculate the correct dose of the medication yourself and 'read between the lines' of what it might say on the bottle so to speak. Dosing doesn't just relate to the volume of water in your pond, it relates very significantly to the water chemistry a well as other factors - and that is why all of our recommendations for parasite treatments are based on the use of generic medications rather than the pre-prepared/pre-mixed versions.



All about Parasite Treatments


On our Koi parasites page, we have attempted to explain how to identify parasites and included a list of recommended treatments for each parasite concerned. However it is important to ensure each chemical is used correctly to ensure successful eradication of any parasites, and it is also vital to understand what can influence success or failure when it comes to treatment.


Like many koi related questions, there is no definitive ‘right’ answer to the issues posed here. Recommended treatments for parasites can vary according to pond conditions and environmental issues, for example:-


  1.  Pond temperature
  2.  Pond water hardness and pH.
  3.  Pond inhabitants
  4.  Condition of koi to be treated.
  5.  Single or multiple parasite infestations
  6.  Condition of your filters.
  7.  Manufacturer of treatment.


Let’s try and explain


Pond temperature


Some chemicals won’t work at low temperatures and some don’t work so well at high temperatures either! Potassium Permanganate is one of the only effective parasite treatment to use in cold water and works well at any temperatures. However at temperatures over around 18 deg C (and upwards) extra aeration must be used with Potassium of you may find you suffocate your fish since this chemical strips large amounts of Oxygen from the water.


In addition as the pond water temperature changes, so does the life cycle of the parasite. As you may know, generally most treatments have to be repeated in order to kill all stages of the parasite concerned. This is because most chemicals cannot kill parasite eggs and you must wait until the eggs hatch before retreating and killing the live parasites. If you leave your repeat dose too long the parasites will have hatched and the hatchlings will have laid eggs again! Too little time between treatments and eggs will be present and the treatment will only be partially effective. So how long do you wait? Well life cycles are influenced by water temperature. In cold water temperatures some parasites take a month to complete their life cycle and some may not complete at all. At high temperatures a complete life cycle can be completed in 3-4 days. Tricky isn’t it?


We know that one of the biggest problems within our wonderful Koi hobby is that there is not enough information available to the average hobbyist about parasite life cycles – most of the information you read in Koi books and on-line articles is too generalised. However there are now a number of good Koi books , and DVDs available specifically on fish diseases, and whilst these are not specifically aimed at koi, most cover the subject of parasites in much more detail. 


Sudden changes in temperature or indeed other water parameters can trigger an outbreak of parasites. For example, Whitespot, which is endemic in many fresh water species, is a cold water parasite commonly triggered into action by moving fish from warm to cold water or simply by rapid changes in a koi's environment.


Water hardness


Water hardness is one of the most significant factors that can influence your decision on the correct dose of most commonly used Koi medications. Many chemicals work differently as the water hardness and pH varies. For example the oxidising agents, Chloramine T, Potassium Permanganate and Virkon Aquatic are not nearly as effective in hard water as in soft water. For example , the recommended dose rates for Chloramine T is between 1.5 and 2 gms per ton (1000 litres)– but this is the dose rate for soft water and low pH. As the water gets progressively harder. the required dose increases until with very hard water, and with a high pH, the recommended dose rate can be up to 5 gms per ton! Guess what happens to your filters if you use it at that rate? This is one of the reasons that whilst Chloramine T is a very good disinfectant, it is not always effective at eradicating targeted parasites unless you understand the water chemistry issues involved. The same applies to Virkon Aquatic - where the dose for very soft water is around 2gm per 220 gallons, but for very hard water is around 10gms per 220 gallons - a five fold difference!


Even when using Malachite Green, Formalin, Acriflavine or Potassium Permanganate water hardness can have a significant impact:- Potassium is more toxic in soft water and high pH, Acriflavine is more toxic in soft water and is not advised if the DKH (German Degrees of hardness) is below 6, and the dosing of both Malachite and Formalin can be increased by up to 20% in very hard water to ensure their effectiveness. That is why it is imperative that you understand the basics of your water chemistry before you apply any medications to your pond.


Pond inhabitants.


You simply can’t use certain chemicals in your pond if you have goldfish, Orfe, Tench or Rudd. As the chemicals will kill the fish as well as the parasites!. Ones to watch are the Organophosphates, Masoten, Dimlin and Supaverm.


If you do have other species present, you may therefore have to revert to a less effective (but safer) remedy with which to treat the pond – or move your other fish to a temporary home whilst treating your koi.


Condition of your fish.


Sounds obvious I know. If you koi have parasites they are not going to be in prime condition are they! What I mean by this is that often older inhabitants of koi ponds won’t take so kindly to chemicals as some of the more sprightly inhabitants. Often, old koi will have sustained some gill damage throughout their lives as a result of periods of living in an artificial environment where water quality was not always as good as it should have been. When subjected to some of the more aggressive chemicals, older koi can succumb whilst the younger koi are fine.


If you have some older koi that might be at risk then you may want to resort to a less aggressive chemical. Potassium Permanganate is the most likely to cause concern as this chemical can easily damage gill tissue.


Single or Multiple infestations.


A complex issue to get into but why have your koi become infested with parasites in the first place? It (they) could have been introduced into the pond with parasites. They could have inherited them from an existing pond inhabitant. Poor water quality may have stressed the koi which will then have attracted parasites, the fish could be suffering with some other form of illness which debilitates it causing parasite attack.


Parasites are opportunistic. Healthy koi are normally able to keep parasite numbers under control. Most koi carry some parasites just like dogs and cats carry fleas. Healthy mucus on a koi damages parasite mobility and koi will produce more mucus (sometimes to excess) when parasites are present in numbers.


If your koi has attracted parasites because it has become ill or stressed, it is very common to find more than one type of parasite present on a scrape. The choice of treatment is therefore more difficult, as you may have to use one chemical for one parasite and another for a different one. Which do you use first? Try and kill the most dangerous parasite first. The most dangerous one is normally the one present in higher numbers and to establish this, you may have to take more than one scrape from the same fish and take a scrape from a second fish to see what you find. Never, under any circumstances mix different chemicals together unless recommended by the manufacturer. You may kill more than the parasites.


Condition of your filters.


If you have koi with parasites, you will most commonly need to treat with Potassium Permanganate, ChloramineT, Malachite and Formalin or Acriflavine . All of these chemicals will affect your filters, but to different degrees. If you have a mature filter this will be of less concern, but if you have an immature set up – less than 6 months old, you may wish to take more care in your choice of treatment so that you do as little damage to your filter as possible.


Probably Chloramine T will damage filters more than any other chemical, then Potassium, but again effects will vary according to specific circumstances. In contrast, Acriflavine, as an example, does not, as far as I am aware, damage filters, and therefore may be your first choice of treatment where your filters are immature.


Manufacturer of drug.


The more you read, the more confused you can become! Different publications may print differing dose rates for the same chemical which only serves to confuse more. However, different manufacturers package the same chemicals with differing strengths so you will always need to follow the instructions on the bottle and NEVER from a book or article as this may refer to a different strength of the chemical. Acriflavine, Malachite, and Formalin are sold commercially in different strengths so use caution when using.


Confusing - isn't it? Summing up, all parasite treatments are chemicals which we would not wish to add to our pond water at all unless absolutely necessary. Most have side affects and all should be used with caution. Normally the correct dose of the correct chemical repeated at the correct frequency will eradicate the parasite. So why sometimes do we report that our treatment has not been totally effective?


Normally the reasons are fairly straightforward:


  1.  We don’t know the correct volume of our pond
  2.  We ‘guess’ at the identity of the parasite (i.e. no scrape to check) or worse we wrongly identify the parasite detected. This is all too common as some of the Protozans look very similar.
  3.  We use a chemical which is inappropriate for the pond temperature and water hardness
  4.  We use the wrong dose (don’t check the strength) – or simply can’t measure it accurately!
  5.  We only use one treatment or use two at the wrong frequency to catch the life cycle.
  6.  We forget to switch of our UV – thus causing the chemical to break down too quickly in the water
  7.  We use dips rather than pond treatments.
  8.  We try to protect the filter by switching off the main filter pump for a few hours.


Please do always switch off UVs or Ozone generators and protein skimmers as these can break down chemicals in the water very quickly and render your parasite treatment useless. When using chemicals, always ensure you know the pond gallonage, and weigh out powdered chemicals using a gramme balance to ensure correct dosing. Similarly when using liquids, measure the correct dose accurately with proper measuring cylinders or syringes.


In our experience, dips are rarely effective in treating outbreaks of parasites as they are very stressful and the danger of overdose is very real as it is virtually impossible to accurately calculate the chemical dose required in a few gallons of water, or to measure the time that the Koi has to be dipped before removal. Probably more Koi are killed by inadvertent use of dips than anything else. You must always treat the pond if parasites are found on one Koi as this means that there are parasites in the pond, and almost certainly therefore on other Koi. Always therefore treat the entire system, including the filters when using chemicals to treat parasites and never switch of your main filter pump(s).


The other very important thing to remember when choosing your medication is how long each treatment remains effective in the pond relevant to the life cycle of the particular parasite in question.


The chemical must obviously remain active in the water long enough to kill all juvenile and adult parasites and the time required varies.


Chloramine T is only effective in the water for around 4 hours and the dose is therefore normally repeated daily or every other day to ensure adequate kill rates. It should also be used in the evening rather than in the morning as UV light breaks down this chemical very quickly indeed.


Potassium is effective for at around 6 hours in the water whilst Malachite and Formalin are normally effective for 2 or 3 days. Remember however that these time vary considerable with temperature and the amount of UV light hitting the water.

In contrast, Acriflavine is active in water for well over a week, and Virkon Aquatic for 4 to 5 days


In conclusion, before treating your pond with any chemical for suspected parasite infestation, you should take steps to identify the culprit(s) and then choose the most appropriate remedy depending on the factors discussed above. Diving in too rapidly and treating with the wrong chemical at the wrong time/dose/frequency can do more damage than not treating at all.


In conclusion, our preferred parasite medications are:-


1. Whitespot - Acriflavine with salt at 1/2 ounce per gallon. One treatment only. 2nd favourite, Malachite Green plus salt at 1/2 oz per gallon. Two treatments required. or - Malachite and Formalin. Two treatments required. Use above 54 Deg F (12 Deg C.)


 2. Costia - Acriflavine with salt at 1/2 oz per gallon. Temperature 56 deg F or above. 2nd Choice Potassium Permanganate plus salt at 1/2 oz per gallon. Any Temperature


 3. Trichodena - Acriflavine with salt at 1/2 oz per gallon. Temperature 56 deg F or above. 2nd Choice Potassium Permanganate plus salt at 1/2 oz per gallon. Any Temperature


 4. Chilodonella - Acriflavine with salt at 1/2 oz per gallon. Temperature 56 deg F or above. 2nd Choice Potassium Permanganate plus salt at 1/2 oz per gallon. Any Temperature


 5 Flukes (Body and Gill) - Fluke Solve or Fluke S - (both treatments contain Praziquantel) at recommended dose. Repeat if required. Supaverm at 5-10ml per 1000 gallons only.

3rd choice - Potassium Permanganate at 2gm per 220 gallons with salt at 1/2 oz per gallon. Max temp 20 deg C.


 6. Argulus (Fish louse), Lernaea (Anchor Worm) and Leeches - Lice Solve at recommended dose. Dimilin at 1gm per 220 gallons. Use only once. Use above 54 Deg F ( 12 Deg C.)


Always use latex gloves and a mask when using powdered chemicals so that chemicals cannot be ingested or come into contact with bare skin. .





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