The Power of Purple

*This article is a revised and updated version of the original article “Steps in a Crisis”, by Matt Lyon, which originally appeared in the Nov/Dec, 2003 issue of The Goldfish Report

In this article, I would like to describe some of the commonly seen problems affecting goldfish, and list the most probable causes of these symptoms. While there are many commercial treatments available for the treatment of goldfish diseases, some of these treatments may do more harm than good if used incorrectly, or if the wrong treatment is used for the presenting illness. Aside from doing a simple water change or correcting pH or temperature parameters, one of the most effective and flexible treatments available to goldfish keepers is Potassium Permanganate (PP). In this article I would like to focus on those conditions where Potassium Permanganate would be a useful treatment, and how it should be applied. Considering the toxicity of some aquarium medications such as Copper Sulphate, Formalin, Malachite Green and Trichlorfon (Trichloracide), PP is relatively non-toxic, and generates no hazardous by-products. When used correctly, PP is very effective and offers many advantages over these other treatments.


As with many other facets of our lives, preventing a problem with our goldfish is far easier than trying to solve one after it arises. Even the most inexperienced hobbyists have at their fingertips all the information needed to successfully keep thriving, healthy goldfish.

Step 1:

The first step in solving goldfish problems is prevention. This starts at the pet store, importer, or breeder’s facility. When purchasing fish, select only healthy looking fish from healthy tanks. Even apparently healthy fish from a tank containing sick fish are likely harboring harmful parasites, protozoans and bacteria. Prevention can also take place at home by properly quarantining all new arrivals, and disinfecting nets, hoses, and other implements that come in contact with more than one tank or pond. A quarter cup of chlorine bleach in a 3-4 gallon bucket of water is recommended as a half-hour sterilizing bath for disinfecting equipment. For disinfecting nets and other more delicate implements, use Benzalkonium Chloride or one of the commercial net disinfectants.

Hobbyists who regularly test their water will be more likely to maintain good water quality. By testing regularly, you will have a much better feel for when water changes need to be performed. Eventually, you may learn to associate other cues, such as water appearance and smell, but until you are confident in your judgement, it is best to just use test kits. The term “water quality”, generally refers to the levels of nitrogenous wastes: Ammonia, Nitrite and Nitrate. The delicate balance of these chemicals is referred to as the Nitrogen Cycle. If you are not already familiar with the Nitrogen Cycle, please research this topic, as a basic understanding of it is crucial to your success as an aquarist. In addition to carefully monitoring nitrogen levels, the aquarist should also be aware of pH levels (especially with well water which is not pre-buffered to a safe level). A good pH range for goldfish would anywhere between 6.8 and 8.0. Generally, slightly harder and more alkaline water is preferred. My well water is very acidic, and runs at < 6.0 pH. I remedy this by adding baking soda, but too much baking soda will also cause problems by making the pH rise too much. Similarly, green water (caused by planktonic algae) will also cause large swings in pH, making pH rise during the daytime, and then drop suddenly after dark when photosynthesis stops and the respiration of the algae and fish release CO2 back into the water. This can cause a stressful and dangerous situation for your fish. Also, if you happen to have a home water softening system, your goldfish might fare better if you bypass the water softener before filling your tanks. If you cannot bypass the water softener you will probably have to add a buffering agent to the water to make the fish happy. Every year, people get new fish into their hobby and find out that the softener water is killing their fish. A certain amount of carbonate hardness (KH) is essential for maintaining a stable pH, and it also assists with maintenance of the biological filter. For this reason, Reverse Osmosis (RO) water should never be used alone. In fact, it is not recommended for goldfish at all, and is best reserved for certain soft and acidic-water loving species.

Have you ever noticed your fish looking stressed, and the water appearing slightly cloudy, but all water quality readings were in the normal range? The affected fish might have a couple of reddish blotches on their bodies, red streaks in their fins, or slightly swollen and open gills? In this situation, you are more than likely dealing with a bacterial and/or fungal bloom, and excess organic waste in the water. This situation is more common in warm and poorly oxygenated water, and may occur shortly after feeding with a rich food that breaks apart easily and fouls the water; in particular, gel foods that are not well bound, or where the gelatin begins to soften in very warm water. This can also be seen after spawning where a large amount of milt and eggs are expelled into the water. All of the rich organic material in the water will give rise to a sudden bacterial bloom which will in turn consume Oxygen and cause fish to gasp at the surface. If not caught quickly, the aggressive bacterial growth can also damage tissues such as fins, gills, and the skin. In the remainder of this article, I will explain how to diagnose goldfish health issues, and offer advice on using Potassium Permanganate to reverse or cure most common problems.

Step 2: Observation of a Problem and Diagnosis

Regular close observation of your fish is not only enjoyable, but very important. Watching how your fish behave on a daily basis will help you recognize abnormal behavior as early as possible. In general, ailing goldfish tend to become sluggish and isolated and often have reduced appetites. Refer to the list of 9 major symptoms shown below and find the one that best matches the behavior/condition of your fish.

The list below is not comprehensive, but covers the most common problems observed with ailing goldfish, and the primary potential causes of each. For problems not covered by this article, there are many books already available that set forth proven remedies.

  1. Fish is Isolated and Not Eating
    1. Poor Water Quality
    2. Flukes
    3. Costia
    4. Bacterial Infection
  2. Fish has Red Blotches on skin and/or Fins (may or may not be eating)
    1. Poor Water Quality
    2. Bacterial/Fungal bloom
  3. Fish has Shredded Fins
    1. Extremely Bad Water Quality
    2. Protozoan Parasitization (commonly Chilodonella)
    3. Injury from spawning, or physical entrapment
  4. Fish is “Panting” on the Bottom
    1. Flukes: Diagnosis Notes, Inspect the fish’s gills by capturing by hand and gently prying open the gill cover. View under adequate lighting. Gills that are unhealthy will be mucus-coated, entirely pale, have pale patches, or be shredded. Healthy gills are a deep red color.
  5. Fish has clamped fins, Excessive slime production, and web-like hemorrhages on skin
    1. Protozoal Parasites, commonly Chilodonella, Costia, or Trichodina.
  6. Fish has White Spots or Specks on Skin and/or Fins (appearing like grains of salt)
    1. Ich-Diagnosis Notes: If the white specks are isolated on the gill plates and/or first rays of the pectoral fins, you are likely observing the breeding tubercles of a sexually mature male.
  7. Fish Dying Every 3-4 Days for no Apparent Reason
    1. Flukes
  8. Fish’s Body is Swollen With Scales Sticking out All Over (Dropsy) (PP treatment not indicated)
    1. Internal Bacterial Infection
    2. Advanced Kidney Disease (water retention)
  9. Fish is Floating Upside Down (PP treatment not indicated)
    1. Poor Water Quality
    2. Digestive System Impaction (constipation)
    3. Advanced Kidney Disease (water retention)

Step 3. Change Water and/or PP treatment

Often times, worried fish keepers will assume that their ailing fish have some sort of invisible “infection” or parasite, and will rush treat with an antibiotic or toxic chemical “cure-all” sold by their local fish store, only to find that the treatment was either ineffective or actually seemed to hasten the demise of their fish. It is most important to remember that environment alone, i.e. poor water quality, pH or temperature extremes, or the presence of chlorine/chloramine or other toxins can cause many of these symptoms, and a simple water change may be the best course of action. Only after performing a water change and confirming that the water quality and parameters are suitable should any chemical or medicinal treatments be considered.

Potassium Permanganate (KMnO4) is a readily available chemical that has many medical and industrial uses all of which exploit its strong oxidative properties. It is most often used in water treatment facilities to control bacterial growth and associated odors, as well as breaking down sludge buildup in plumbing pipes, and removal of iron from drinking water. It is a salt consisting of Potassium and Manganese ions and is a strong oxidizing agent, similar to Hydrogen Peroxide or Chlorine. It is most often sold as small crystals, or in a dry powder form, but is also available as a concentrated liquid for aquarium use. Although it can be hazardous due to its strong oxidative properties, it has the benefit of being non-toxic after being reduced, and it gives off no toxic by products. Additionally, PP does not expire when stored correctly, and it can be used at different dosages for different effects. In solution, it is a pinkish purple color that varies with the concentration, however, once reacted, it turns the water a yellowish brown color. This color change is very convenient for visually assessing the whether the treatment is still active or has been reduced (neutralized). And finally, it is somewhat forgiving in the event of an “overdose”, since it can be quickly and completely neutralized by the addition of Hydrogen Peroxide (H2O2) which will stop the Oxidative process immediately. Fortunately, H2O2 is also environmentally friendly.

Water should be a light pink/purple color at the 2mg./L treatment dosage

It is important to consider the mode of action of PP and compare it to other available treatments. Basically, an oxidizing agent can kill bacteria, fungi and protozoa by disrupting the cell walls or membranes of microorganisms, as well as causing DNA damage and affecting important molecules such as enzymes. In order for the oxidation to occur, the oxidizing agent must have direct contact with the surface of the cells. For this reason, PP is only recommended for treatment of surface infections or parasites (on the fish), or disinfection of the water and aquarium contents. Any internal infections, such as Dropsy, or intestinal parasites, would not likely be cured with a PP treatment. The limit to the strength of the PP treatment will be the dosage that will be effective against the pathogen, but not so harsh that it will do serious damage to the sensitive gill tissues. Simple organisms such as bacteria, protozoa and flukes will be very susceptible to the PP treatments. However, some microorganisms can burrow into the slime coat and may require more stringent dips or baths for complete eradication. The same goes for fish lice and anchor worms, which are higher organisms and will require a stronger dip or bath to remove them. The juvenile stages of these parasites that are free swimming will be more susceptible to the treatment. When treating small fry, it is important to consider that their developing gills and other tissues are still very fragile, and they might not be able to survive a PP treatment. Especially in the case of flukes, it may be best to treat very small fry with a fluke-specific treatment such as Praziqunatel (Prazi-Pro), because this will be the least risky treatment for a very specific parasite.

When treating with PP, you should first decide whether you are simply trying to reduce suspended bacterial, fungal, or algal growth, or whether you are trying to eradicate a specific pathogen, such as flukes, Costia, Chilodonella, or Trichodina. In the first case, the water may appear cloudy and have a bad smell, or the fish may be showing a few reddish blotches on their skin. In this situation, it is helpful to remove as much organic debris from the aquarium as possible before beginning the treatment, as the organic load will be oxidized by the PP, and in turn, it will neutralize or reduce the PP, possibly shortening the effective treatment time. The largest concentration of organic material is usually found in the filter, so the filter should be removed and cleaned manually. While it is sometimes okay to expose the filter to the PP, this may temporarily reduce the population of “good” bacteria in the biofilter, so it may be best to bypass the filter all together. For this treatment, a standard low dose of 2mg/L is sufficient, and the water should remain a light pink color for a full four hours. If the water becomes brown or yellow during that time, the treatment should be repeated at a 1.75mg/L dose to maintain the pink color for a full four hours. To de-color the brownish water at the end of the treatment, simply add 1 tsp H2O2 per 20 gallons. It is important to note that Sodium Thiosulphate, the active ingredient in dechlorinators, will also deactivate a PP treatment. For this reason, it is advisable to perform the PP treatment before doing a water change, or at least before adding water conditioner. Daily water changes for at least a week following treatment will keep nitrogenous wastes at harmless levels until your biological filter recovers. Be sure to test ammonia and/or nitrite/nitrate levels daily until you are certain that the beneficial bacteria in the biofilter have been restored.

In the case of a confirmed parasite infection, it will be important to treat the entire tank and filter at a higher dose to ensure that all stages of the parasite’s life cycle are destroyed. The aquarium and filter can be treated with a higher concentration of PP, perhaps 10mg/L, however the fish should first be removed and treated separately in a 5 gallon bucket with strong aeration. Since the stronger concentration may damage plants and kill snails, any desirable specimens should be removed before treatment; most plants will survive a lower dosage treatment, but snails may not. For the fish, the treatment can be either a low dose for a longer period (e.g. 2mg/L for 4 hours), or a higher dose for a shorter period (10mg/L for 30 mins). If the fish are already appearing weakened and are having difficulty breathing, then it is best to use a lower dosage so that the gills are not further compromised. Another bucket containing clean water should be kept nearby, so that fish can be quickly removed from the treatment bucket once the treatment is over, or if they begin to show signs of distress. If a separate, clean aquarium is available, the treated fish may be moved to that tank to avoid the possibility of re-infection. Otherwise, the affected tank should be drained and refilled after the PP treatment and then the fish can be returned. Following the PP treatment, fish can be treated with salt at a concentration of 1 Tbsp per 5 gallons for a week or so, or until they have fully recovered.

To make a Potassium Permanganate Stock Solution, use a 1 liter bottle (mark the side with a Sharpie marker at 1 liter volume, then add a level ½ tsp. (3,000 mg) of dry Potassium Permanganate crystals (PP), and fill with distilled water to the 1 liter mark. This will be your stock solution for treating tanks, and tubs. Similarly, you can use a 2 liter soda bottle, and 1 tsp of PP.

Liquid and dry measuring tools: Pitcher, measuring cup, and measuring spoons, distilled water, plastic bottle with lid, and Hydrogen Peroxide (3%) for terminating treatment, and decoloring water.

The desired treatment concentration (in the aquarium) for a 4-6 hour treatment is 2mg/L. The conversion between liters and gallons is 1 US gallon= 3.785 liters, so the equivalent dosage would be 2mg/L x 3.785 liters= 7.5mg/gallon, or 75mg/10 gallons.

So how much stock solution is needed to treat a 10 gallon volume of water?

1 liter of stock solution contains 3,000mg PP, so what volume of stock solution will contain 75mg of PP?

To solve, divide 75mg/3,000mg = 0.025 liters (or 25 mls). You can simply use 25mls of stock solution for each 10 gallons of water to be treated. This volume can be easily measured with a liquid measuring cup. It is safer to use this stock solution than a more concentrated one, because small measuring errors are magnified with a more concentrated stock solution, making it easier to accidentally use too much Potassium Permanganate, and potentially injure or kill your fish.

To neutralize (deactivate), use 3% Hydrogen Peroxide (H2O2), which is the brown bottle type sold in drug stores. Dose at 1 Tbsp per 50 gallons, or roughly 1 tsp per 20 gallons. The Hydrogen Peroxide will also clear the water, removing the brownish yellow tinge from the Potassium Permanganate treatment. Any residual H2O2 will remain active for up to 2 days, so a large water change is recommended before attempting another PP treatment.

In summary, do your best to prevent fish health problems with careful quarantining and regular maintenance, but be prepared to act with the safest and least toxic remedies first. Often, having a ready-to-go stock solution of Potassium Permanganate can ward off bigger problems down the road.


A couple more suggestions… When handling PP, always wear old clothes and plastic gloves. If you splash the solution on anything, it will be stained forever. Never add PP to Formalin containing treatments, or water containing Formalin, since this will cause a chemical reaction which releases poisonous Formaldehyde gas.


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