Potassium Permanganate (KMnO4).

Here is an oldtimer's standby with some very circumscribed aquarium uses. If you aren't cautious, you can do more damage with potassium permanganate than with almost any other chemical in your arsenal. At first appearance, potassium permanganate sounds like a superweapon. It is an oxidizing agent that reacts with suspended and dissolved organic molecules. Its caustic action also oxidizes the cell walls of bacteria. That gives it antiseptic properties, if you're dealing with a wound or with bacterial ulcerations. And over sixty years ago, William T. Innes was recommending KMnO₄ at a half-grain to the gallon, as an algicide to clear "green water," for it acts against the unprotected cell walls of algae too. By decimating planktonic protists it will also clear hazy cloudiness in the water, rather the way gunfire will clear the schoolyard. Potassium permanganate reduces high levels of organic substances, especially in koi ponds where water changes are impractical, and it's effective there on bacterial gill infections and "Columnaris Disease." In an aquarium, probably you should be doing water changes instead. At higher concentrations KMnO₄ is effective against monogenetic trematodes or "gill flukes," Trichodina, ciliates during their vulnerable free-swimming stage, and it counters some fungal infections too. Potassium permanganate kills snails. It kills other mollusks too; Baltimore's water authority uses KmnO₄ in the city water mains to prevent the possible infiltration of zebra mussels.

So why isn't potassium permanganate a godsend? Because using it is a little like using chlorine! Like chlorine, potassium permanganate is utterly unselective in its action. Oxidizers react with any organic: bacteria, protists, algae, DOC and particulate detritus-- but also the delicate epidermis covering fish gills. And the toxic level of KMnO₄ is only slightly higher than its therapeutic level of 2ppm. For one thing, it will decimate the desirable nitrifying bacteria in your sponge filter as fast as it acts on unwanted bacteria elsewhere. And some plants are extra sensitive to KMnO₄: after a dose of potassium permanganate, Vallisneria can melt away as if it were Cryptocoryne.

How does potassium permanganate work? It dissociates in water to release an ion of potassium and a permanganate ion, MnO₄¯. That reactive molecule is unstable, one reason why you should purchase KMnO₄ in powder or dry tablet form and make up a fresh stock solution. In a fresh solution, the permanganate ion releases a caustic oxygen molecule--—O₂--— when it encounters most organic materials, including desirable humins and polyphenols. Though it's not a primary disinfectant, KMnO₄ oxidizes cell materials of a wide range of bacteria, cyanobacteria, fungi, algae, protozoa and viruses. The oxidation acts like bleach. The reaction leaves manganese oxide, MnO₂, which precipitates out, perhaps temporarily browning the glass of the tank. You can see the reaction happening, as the magenta pink color of unreacted KMnO₄ oxidizes first to a rosy tea color, then to amber and brown; the time it takes to spend itself depends on the concentration of dissolved organic matter. In fact a rough-and-ready field test for dissolved organics measures the time it takes for KMnO4 to completely oxidize in a water sample.

Potassium permanganate oxidizes iron and manganese, too, making them precipitate out. This effect might be undesirable to some planted-tank enthusiasts, who add iron to the water, but others associate iron with algae production. There is a slight reduction of alkalinity associated with the reaction, through acid (H⁺) production, but not enough to consider in the context of water softening.

Sometimes fishes overdosed with potassium permanganate show the rusty brownish coating on their fins., as the glass jar in which you make up your stock solution will be stained with brown magnesium oxide. That liberated O₂ which oxidizes any organic material it encounters, can burn your fishes' gills as thoroughly as chlorine. The gill lamellae may form scar tissue that permanently affects their efficiency, leaving your fish forever gasping for oxygen. "Scaleless" fishes that are sensitive to all medicaments, are especially vulnerable to potassium permanganate. In years gone by, goldfish were dipped for up to 90 minutes in a potassium permanganate solution; they would survive, apparently fine, but die from gill damage as much as several weeks afterwards. So the practice was discontinued.

A virtue of treatments using potassium permanganate is that the color of the solution gives you an indication of the strength of the dose (water should never be more than pale magenta pink) and lets you follow the action and know whether the effect is spent and decide if you should re-dose.

Some cautions. In dry form, as a black-purple cake or pill, potassium permanganate is a serious skin and eye irritant and is potentially fatal if swallowed. The material safety data sheet on KMnO₄ should be carefully read. Your stock solution should be about 1/8 teaspoon of dry crystals in a pint of water; the traditional "pinch in a pint." You'd use this stock solution in a concentration of 5 tablespoons per 10 gallons to make a short-term bath. Or you could pre-dissolve 1/8 teaspoon of crystals in 3 gallons of water as a three-minute dip to kill external parasites. The potency of potassium permanganate depends on the organic content of the water; in addition, it's good to be aware that KMnO₄ is more potent in water with a low pH. Besides organic content and pH, its toxicity to fishes also varies according to hardness, and even depends on temperature. Common dosing levels range from 2 to 4 ppm.

Why would you ever use an agent like this in an aquarium? KmnO4 does have limited uses. As a 90 minute dip for plants, for example, a mild solution of potassium permanganate will eliminate snails and other unwanted hitchhikers. If you are breeding fish whose eggs are very sensitive to organics in the water, you might pre-treat the spawning water; before introducing the fish, you'd wait for the color to oxidize. Or you might use it simply to test for organic pollutants. As a general disinfectant potassium permanganate can be made into a 10 ppm solution. Cautious treatments are made at daily intervals, followed by water changes. When the water remains magenta pink for several hours, potassium permanganate has oxidized most of the available dissolved organics. Fish should not be subjected to this regimen.

Kordon markets a 3.84% solution of potassium permanganate as "Permoxyn" if you can't get it cheaper in crystal form, either at the drugstore or among water conditioner supplies at your local HardWarehouse or wherever. Read up on Permoxyn at the Kordon site. The company makes this disclaimer: "This product should only be used when there is evidence that excessive and/or dissolved organics are present." Permoxyn is "not intended as a medication or chemotherapeutic agent." That should keep the FDA off their back. Jungle's "Clear Water Extract" also contains potassium permanganate.

KentMarine markets Poly-Ox, a solution of "manganic acid salts" (manganic acid being H₂MnO₄) that works very much like potassium permanganate (KMnO₄) to oxidize organics. Before you consider using it, read the distributor's copious cautions at the KentMarine website .

H₂O₂ as an antidote. If you want to quickly deactivate KMnO₄, you can do it with hydrogen peroxide, which is another caustic oxidizer with antibacterial properties. The H₂O₂ should be at a concentration of 5ml/20 gallons: it will work in a few minutes. You should be aware that KMnO4 inactivates formalin and malachite green; the potassium permanganate will act as an antidote if they are all used together. AmQuel will also react with Kordon's Permoxyn. U. of Florida Extension Services recommend KMnO₄ for commercial fishponds as a safer substitute, though pricier, when copper sulfate is too unpredictably toxic, due to lack of alkalinity in the water.

"Potassium permanganate can be applied at a concentration of 2 mg/L, which will result in a purple-pink color of the water. If the water turns yellow or brown in less than 8 to 10 hours, then the treatment should be repeated. Usually, a maximum of three applications (2 mg/L each) is recommended during any one treatment (maximum concentration of 6 mg/L)."

Dr. Erik Johnson explains the action of KMnO₄ in two articles at www.koivet.com, working from the perspective of its use in ponds. (Click on "Medications & Doses" in the menu). He explains it carefully but adds, "If you do not understand this regimen, do not use it."

But Dr. Roddy W. Conrad's "Protocol for using Potassium Permanganate in the treatment of Koi and goldfish" at 2 ppm rather than the more common 4 ppm, archived at the www.arborman.com site, is the best PMnO₄ article on the Web.

Potassium permanganate is also covered in a 1999 E.P.A. guidance manual "Alternative disinfectants and oxidants" (Do a www.google.com search for the title's key words if the link is ineffective.) The document very briefly touches on the main effects of potassium permanganate.