Potassium permanganate effective, but potentially dangerous treatment for parasites

Potassium permanganate and fish disease

Potassium permanganate is a useful fish disease treatment; acting against a range of protozoan parasites including Trichodina, Costia and Chilodonella, as well as monogenean flukes Gyrodactylus (skin flukes) and Dactylogyrus  (gill flukes). In addition to being an effective anti-parasite treatment, potassium permanganate can also assist with bacterial gill disease and bacterial disease such as skin ulcers. It will also oxidise dissolved organic matter, reducing the biological oxygen demand and improving water quality and clarity. All sounds too good to be true!

With regard to ornamental fishponds, it is mainly used in koi ponds because of dosing difficulties in heavily planted ponds.

Effective against Trichodina

My own experience is that in most cases it is very effective against Trichodina, clearing the problem quickly. Against this particular parasite it would, in most circumstances, be my first choice. I have had mixed results against flukes and Costia, but this is possibly due to the excess mucus often produced by these parasites providing protection.

A powerful oxidising agent.

As with many disease treatments, potassium permanganate is not really a medicine. It is a caustic alkali that, at high doses, can cause serious corrosion of delicate tissues such as skin and gill. In ponds it works as an oxidising agent in a similar, although milder, fashion, to household bleach! It is a very reactive chemical, reacting with organic material.

a powerful oxidising agent

Oxygen is usually seen as a benign substance. However, oxygen atoms – as released in some oxidising reactions – are extremely reactive. One common reaction involving oxygen is fire and fireworks. A slower, but just as destructive reaction is rust, in which iron is oxidized.

All oxidising agents should be used with extreme caution – double and treble-check the pond size and calculated dosage.

During such reactions the manganate ion, MnO4– loses two oxygen atoms and is reduced to insoluble manganese dioxide MnO2. The ‘lost’ oxygen atoms react aggressively with other organic molecules, altering their structure and properties. It is these reactive properties that kill bacteria and parasites such as Costia, Trichodina and flukes.

Be careful

Although it is a very useful disease treatment, its reactiveness with organic material can make it a difficult and potentially dangerous treatment to use. It will react readily with any organic matter. If the pond has any particulate organic material, such as algae, detritus, or dissolved organic compounds, then much of the oxidation reaction will take place with these organics rather than the parasites or bacteria we are targeting. For this reason potassium treatments will not be effective in green water.

Therein lays a major problem with its use. We need to calculate a dose that will leave an appropriate residue (1.75 mg/litre) of unreacted manganate ion after any such reactions. It is only this residual level that will be effective against bacteria and parasites such as Costia, Trichodina and flukes.

Clearly, the level of dissolved and particulate organics will vary considerably from pond to pond – dependent on many factors. If the pot. perm. residual level is too low then the treatment is liable to be ineffective. If, however, the residual level is too high, then it is likely to be harmful and possibly fatal to fish.

There are two methods to calculate the required doses.

  • The first is to calculate the organic level of the pond water by adding 0,1,2,3,4,5,6,8,10 mg of pot.perm. to separate one-litre containers of pond water (plastic bags will do). After 20 minutes see which is the lowest concentration still retaining a very faint pink hue. (Or alternatively add successive 1 mg doses at 20 minute intervals to the same litre of water until the water just stays faintly pink.) If the lowest concentration that just stayed pink were, for example, 2mg/l, then the appropriate pond dosage would be 2 + 1.75 = 3.75 mg per litre.  This method does depend on very accurate measurements. To aid accuracy, a stock solution of one gram per litre could be used, giving 1mg potassium permanganate per ml
  • The alternative is to treat the pond with 2 mg pot. perm. per litre. If it retains a pink hue for 4- 6 hours then the dosage was probably OK. If not, re-dose the pond the following day with 1.75 mg/l and repeat this daily until such times that it does retain a pink hue for 4 – 6 hours. You should not add more than a total of 6-8 mg  to a pond during any course of treatment. If these levels are needed, then clearly the organic level is too high and needs to be reduced by pond cleaning and/or water changes before treatment.

More about potassium permanganate

  • Biological filtration systems should be isolated during treatment, as bacterial flora will be adversely affected.
  • Repeated treatments  may result in cumulative gill damage
  • It is affected by sunlight, so treatments are best carried out in the evening, or on cloudy days.
  • It is best to give the pond a good clean and vacuum before treatment to reduce the amount of organic matter.
  • It is is more toxic at higher pH. In alkaline conditions a solid precipitate of manganese dioxide (MnO2) can form on gills. This can cause respiratory problems by blocking the gills. Additionally, MnOis a strong oxidising agent and presumably in such a situation could cause other gill-tissue damage.
  • Its use against bacterial gill disease has to be balanced against the possibility of further damage from the treatment. It will assist by killing bacteria and parasites in the gills. However, in such situations the gills are often swollen and congested and if a manganate precipitate does form on the gills it could push the fish over the top. This is more likely in alkaline conditions. It would not be my first treatment of choice for this condition unless there were serious parasite complications.
  • It should not be mixed with formalin as this produces toxic formaldehyde gas.
  • It can be neutralized by adding hydrogen peroxide. A recommended level of one litre of 3% hydrogen peroxide (mixed in 10 litres of pond water) will deactivate 20,000 litres of treated pond water.  I have been using just 150 mls. of 9% hydrogen peroxide (available from drug stores) per 20,000 litres pond water to neutralise remaining potassium permanganate residues at the conclusion of treatments. This quickly clears the murky brown water, leaving the pond crystal clear. If too much hydrogen peroxide is used the unreacted residue action may prevent further potassium treatments for a day or two, so it is best to only use hydrogen peroxide at the conclusion of a treatment. If the water gets too murky between treatments (as would happen with a high organic content), hydrogen peroxide could be used to clear the water but it would be advisable to do a good water change before continuing treatments.
  • It is effective against smaller ectoparasites such as Costia, Trichodina and Chilodonella  however its effectiveness against larger parasites such Gyrodactylus (skin flukes) and Dactylogyrus  (gill flukes) is dosage dependent. At residual doses below 1.5 mg/litre, treatments against flukes are not likely to be effective. Short-term bath treatments at higher doses are liable to be more successful.
  • It can be a useful support treatment when treating bacterial ulcers; first by reducing any parasite load and secondly by reducing both the organic content of the water and the bacteria levels. Although it will assist in the healing process, it will not, on its own, cure ulcers.
  • Wear gloves when using potassium permanganate as it will quickly react with the skin leaving a nice deep brown colour that looks as if you smoke about 100 a day!


Pond or tank treatments: residual 1.75 mg / litre. Effective against bacteria and parasites (not larger parasites such as Lernaea  or Argulus ). Can be repeated every 2-3 days – maximum 3 treatments.
Short term baths: Effective against bacteria and parasites (not larger parasites such as Lernaea or Argulus ). Can be repeated daily. Maximum 5 treatments.
Dips: – 20 mg / litre for 20 seconds. Said to be effective against Lernaea or Argulus Dips can also assist in stubborn parasite cases. Only use as a last resort.

See the fish disease section for more details about specific parasites or diseases

Useful conversions are:

ppm = mg/litre            i.e.  5 ppm = 5 mg / litre

mg / litre  x 3.785 = mg / gall (US)           i.e  5 mg / litre = 18.9 mg / gall (US)

mg/ litre x 4.546 = mg / gall (UK)            i.e  5 mg / litre = 22.7 mg / gall (UK)

To convert imperial gallons to US gallons multiply by 1.2

Other useful figures: 

1 ounce = 28.35 grams

1% solution = 

10 ml per litre

10 gram per litre

38 gram per gall  (US) 

45 gram per gall (UK)