Potassium permanganate

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, apparently more effectively at higher pH. Its caustic action also oxidizes the cell walls of bacteria, apparently more effectively at lower pH. That gives it antiseptic properties at <pH7, 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.
Potassium permanganate breaks down 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," a bacterial infection. In an aquarium, although you may read that "it is used to remove organic build-up and odors in tank water", which is its major use in municipal water-treatment facilities, surely you should be doing water changes instead. At slightly higher concentrations, but still never more than about 2 ppm, KMnO4 is effective against monogenetic trematodes or "gill flukes," Trichodina, and 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 pulses of 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 2 ppm. At the therapeutic level, researchers in 1975 found that potassium permanganate did not initiate an ammonia spike; at slightly too high levels  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.
Some uses. Why would you ever use an agent like this in an aquarium? KMnO4does 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.
How does potassium permanganate work? It dissociates in water to release an ion of potassium (K) and a permanganate ion, MnO4¯. Look at that powerful anionic (negative) charge: 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 stable 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 ionic iron and manganese, too, making them precipitate out. This effect might be undesirable to some planted-tank enthusiasts, who are adding iron to the water, but other aquarists 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, somewhat 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 developing action and know whether the effect is spent and decide if you should re-dose.
Some cautions. In dry form, as a black-purple powder, 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.
Purchasing potassium permanganate. Kordon markets a 3.84% solution of potassium permanganate as "Permoxyn",  and Brightwell's "Redoxiclean" has a "proprietary" blend of "manganese salts", if you can't get the pure chemical itself 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, quite an informative page, just what I'd like to read about any treatment or medication.  According to its manufacturer, 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 at 2 ppm 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)." That is a recommendation for ponds, not in aquaria containing small, delicate fish, where 2mg/L for four hours might be safer limits, and a day or two should pass before a repeat treatment.
Dr. Erik Johnson explains the action of KMnO4 in "Potassium permanganate basics" and "Potassium permanganate regimen: against ciliates", both at Koivet, working from the perspective of its use in ponds. 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  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". The document very suiccinctly touches on the main effects of potassium permanganate in the context of municipal water management.