Ich (Ichthyophthirius multifiliis). "I. multifilis" is an alternate spelling. Single-celled Ich is a ciliate, the one that gives all ciliates a bad name. Most ciliates are benign, and some are important to the aquarist: for example, Paramecium, beloved of high-school seniors, is the nutritious ciliate that's a major component of infusoria. Ich evolved in Eurasia; it made its unwelcome North American debut, not by design, at the Chicago Columbian Exposition, 1892-93.
Know your enemy. Be aware of Ich and other parasites as living organisms rather than merely agents of disease symptoms.
Everyone struggles with infestations of "Ick" as a beginner. Some, though, continue with cycle after cycle of Ich, a frustration that should be avoidable. "Uncontrollable or recurrent infestation with ciliate protozoans are indicative of husbandry problems," the University of Florida Extension Service tells you severely. Many of those whose fishes are suffering with Ich don't even want to know it's a ciliate, they just want remedies to nuke it with! But don't scroll down just yet! most problems with permanently eliminating this parasite come from not understanding its life cycle.
Ich has three life-stages. We can begin with the feeding stage that has settled under your fishes' outer skin: the trophont (or trophozoite). The trophont is the only feeding stage (its name contains the same Greek troph="feeding" element familiar in "heterotroph" or "trophic level"), yet it has no mouth. Instead it secretes histolytes to break down neighboring host cells, in order to absorb their contents. The histolytes cause the host fishes' epithelium to thicken, so that the host's own immune reaction provides a safe haven for this "adult" or "mature" stage, where it's protected from medication.
Constantly rotating inside its pustule, the trophont swells to 50 times its original size, becoming large enough to appear to the naked eye, grayish-white, round to oval, as big as a grain of salt. In a few days or much longer, depending on temperature, it is ripe. It sheds its cilia, grows a thickened gelatinous outer shell, lets itself be shed into the fishes' mucus, and drops away as a "tomont."
The trophont does not need to become completely mature. A lab study by T.A. Nicholl and M.S. Ewing at Oklahoma State found that most of the embedded trophonts left the host within four hours of the host's death: it's worth noting, in this context, that the corpse of an Ich-infested fish is a major source of infection.
The released non-feeding tomonts swim for 2 to 6 hours before settling on a substrate. (Nicholl and Ewing found that a light substrate was preferred to a dark one.) Some biologists count this brief interval as a fourth life stage (during which it is susceptible to medication, by the way, according to Dr. Peter Burgess, the resident "fish doctor" at Practical Fishkeeping magazine).
Quickly the free-swimming tomont attaches to a substrate and encysts, as the reproducing stage. This life-stage doesn't eat. Its metabolic clock has been ticking since it left its host fish; now it is spending its stored energy to divide and divide again within the short-lived cyst. The tomont's time-span remains temperature-dependent: at common aquarium temperatures it's a matter of hours to days. (In a chilly koi pond in early spring, the cyst may persist longer.) Ultimately hundreds of microscopic mobile tomites burst from the cyst, even as many as 2000. They quick sprout cilia and start actively swimming about in search of a host. The fully developed "swarmers" are now called theronts (Greek ther- denotes a critter).
The tomites'/theronts' metabolism is also temperature-dependent, but they must find a host within a very few days or perish: at 68°F none survived after 55 hours, according to the pioneer expert in fish diseases Wilhelm Schaperclaus. The gelatinous thin-walled cyst can't survive being completely dried out, an incentive to let your nets dry out completely, if there is Ich anywhere among your tanks.
Only the free-swimming life stages are susceptible to medications. Only the actively feeding trophont can persist "dormant" in the aquarium, though it's never free-living but always attached inconspicuously to a host, perhaps on a gill surface.
Ich is all too easily identified with the naked eye at the final full development of the trophont. (In fact Ichthyophthirius multifiliis is the largest known single-celled parasite on fishes.) As a result, many aquarists don't want to believe Ich is attacking their fish till they actually see the white spots. This is an error. Often a spot or two pass unnoticed, since newly-settled trophonts are too small to see anyway, and especially since early infestations are likely to attach to the gills, where they stay invisible as they grow. If you wait until later stages, badly infested small fishes may be too weakened to save. The other common error, once a medication regime is begun, is that it's often ended as soon as no more mature "white spot" trophonts are visible in obvious places on the fish. It's absolutely essential to keep on medicating till the last encysted tomont has released its tomites, and the last tomite has been eliminated by the medication. Remember, it only takes a single tomite successfully settled into the fishes' epithelium to inaugurate a whole new cycle. As the University of Florida put it, once more, "Uncontrollable or recurrent infestation with ciliate protozoans are indicative of husbandry problems."
Treating Ich. First advice: Catch Ich early. If you notice fishes "flashing" and scraping against rocks as if their gills were being bitten, you have a strong presumption that some external parasite is at work. Labored breathing, loss of appetite, and listless hiding are all signs of more advanced malaise that should be danger signals that something is going wrong for that fish. Don't wait for the tell-tale matured cysts to show on the fishes' fins and body.
I've recently read that at lowered temperatures the vulnerable free-swimming stage can last as long as 96 hours: that's four full days! This might be a problem in a cool outdoor pond, but rather than drag out stressful treatment, you'll surely begin by raising the temperature in the aquarium. Adjusting the setting on the heater will do the job; don't try to raise or lower water temperatures faster than a couple of degrees an hour. Higher temperatures result in speeded-up metabolisms, both for the parasite and for the fish. Above a certain temperature (86°F is often quoted) the heat-sensitive parasite will be stressed. Its life cycle is interfered with, and it might even be killed. But if the host fish is also stressed, high temperatures can be counter-productive. You need to judge your individual species' tolerance for heat; a White Cloud Mountain Minnow or a Corydoras will be more stressed by heat than a Discus. Since at higher temperatures the water holds less oxygen, you might want to lower the water level enough to get some extra splash from the filter return.
I understood long ago how the matured ciliate dropped from the fish and settled on the bottom, enclosed in a protective envelope while it divides into tomite swarmers. But I only recently understood that the theront can settle out on anything: it doesn't have to be gravel or a stone at the bottom: it can be a leaf or a speck of detritus. In this way Ich can be introduced with new plants taken from an infested aquarium, a vector that could easily give the appearance of spontaneous generation of Ich in an aquarium: the Myth That Will Not Die. You can only be sure this won't happen, if there are no fish in the tank your plants came from. Have you noticed that the very best LFS sell you plants from well-lighted but fish-free tanks? You can give new plants a bath with potassium permanganate when you get them home, though that's hard on their delicate tissues. Or, a better plan, just quarantine all new plants for three days, making sure they get warmth and plenty of light.
Of course the free-swimming tomite stage can also be introduced in apparently uninfected new fishes' travelling water: yet another reason to net out your new fishes, after you've acclimatized them, and discard the water.
The trophont or feeding stage attached to the host is also temperature-sensitive. At normal aquarium temperatures it matures in three to four days. At cooler temperatures the whole cycle can be slowed to more than five weeks, as the frustrated pond-keeper can tell you!
Particularly resistant fishes can remain asymptomatic through several cycles of infestation and can act as "carriers" of Ich. What happens is, the free-swimming tomites attach most easily to the gills. The rest of the fishes' skin is protected by a sturdier mucus coating that's constantly renewed, sloughing off all kinds of minute organisms that might settle out. Trophonts that are newly-attached to the epidermis are invisibly small. So a "carrier" fish is simply one that is invisibly carrying Ich, perhaps on its gills. There is no "dormant" independent, long-term encysted life stage separate from a host fish for Ichthyophthirius multifiliis. This is useful to know. You will often hear to the contrary. Dr. Peter Burgess, who took Ichthyophthirius multifiliis as his Ph.D. subject at Plymouth University, mentioned among Ich "old wives' tales" that "It's present in all aquariums." "What utter rubbish" noted Dr. Burgess (in the November 2001 Practical Fishkeeping). Brits don't mince words.
Though they all follow the same life-pattern, there are countless strains of Ich, some harder to kill, some more virulent. Peter Walker alerts me to an article published by Ross F. Nigrelli et al. in 1976, which asserted "probably multiple physiological races exist, perhaps even several separate species (possibly in more than one genus)." Bad news comes in from all over: Ich has traditionally succumbed at temperatures over 85°F, but in the 1990s new Florida "strains" were reported that could survive temperatures as high as 90°, which might overstress most tropical fish. In Oklahoma, university researchers studied a closely-related ciliate, identified as a "strain of Ich," in which the reproducing stage doesn't drop away but remains under the fishes' epidermis and releases the tomites from that secure position. This is a one-shot deal for the trophont, which ruptures and dies in the process. The free-swimming tomites/theronts are still the only stage vulnerable to medication. In this close relation of Ich, the lesions are larger, looking like carp pox, flattened and waxy appearing. There is more at Koivet.
Remedies. Though copper sulfate is used in fishponds, under controlled pH levels, copper is too unstable in toxicity to use in most freshwater aquaria. There are two preferred aquarium techniques for eliminating Ich. (These work against other single-celled external parasites, such as Oodinium, by the way.) One is a long-term salt bath, building up over the first eight hours to a strength of one level teaspoon of common table salt per net gallon of water (and for some salt-tolerant freshwater fishes as much as a tablespoon, the equivalent of three teaspoons), at a temperature that is raised over 86oF. Plain table salt won't affect the pH of the water, but it must be strong enough: the Southern Regional Aquaculture Center reports a case of Ich withstanding a long-term bath of 5 parts per thousand.
Many experienced fishkeepers swear by this traditional salt bath. They prefer it to the other technique, which is far more toxic: a mixture of formalin and malachite green. Which brand you use doesn't matter; just look for a medication that contains only these two ingredients (such as Kordon's "Rid-Ich" or Aquatronics' "Formalite II" or Mardel's "Maracide"). But note that these two ingredients are more powerful in sync than they are separately. That's what "synergy" is, eh! Read up on formalin/malachite green at Fishdoc.
Formalin and malachite green, like potassium permanganate and several other therapeutic agents, react with dissolved organic carbon in the aquatic system. In water that is loaded with organics, their power can be mis-spent on oxidizing organics rather than attacking the protein and polypeptide surface coatings of the disease-causing organisms. This has the effect of lowering the effectiveness of the dosage. So, once you've turned up the thermostat on the heater, a thorough tank cleaning and a good water change, to reduce the level of organics, is always a wise second move, before any medicating begins. People have often noticed that a thorough gravel cleaning makes treatments more effective; generally they ascribe the effect to scooping up of microscopic Ich theronts or tomites out of the gravel, but perhaps the removal of organics is more essential. I feel that rooting around in the gravel to siphon out ciliates is like trying to eliminate carpet mites with your vacuum cleaner: though you may decimate the population, you won't utterly eradicate them.
Potassium permanganate treatments are largely ineffective in eliminating Ich once it is encysted on the host fish. However, a USDA Agricultural Research Service study in 2000, "Prevention of an initial infestation of Ichthyophthirius multifiliis..." found that infestations of Ich could be prevented in fish that had been experimentally exposed to the young Ich swarmers by treatment with as little as 1.0 ppm potassium permanganate. Successful treatment depends on the KMnO4 remaining reactive for four hours, and that again depends on the low levels of dissolved organic carbon.
Herbal remedies. A Melaleuca extract is sometimes recommended against Ich. Kordon manufactures Ich-Attack, a treatment whose active ingredients are naphthoquinones, a range of plant-produced natural phenolic compounds that are produced by many families of plants and some fungi. It is one of three Kordon herbal remedies, offered at aquarium or pond strength.
Fishkeepers realize that some fishes in an aquarium will remain uninfected in an epizootic. So it's interesting to see that these USDA researchers found that Channel Catfish were 33 times more likely to become infected than Blue Tilapia.
Remove filter carbon. Let me remind you again, before you begin any medicating, to take the carbon out of your filter. It's always a good idea, but especially if you have fishes that may be sensitive to medication, to build up towards the full dosage in increments, at three-hour intervals. This holds good for salt baths too. Keep an eagle-eyed watch for signs of distress from heat and medication, such as increased respiration rate. If you see the first signs of stress, immediately do a 25% water change to reduce the dose of medication.
Don't overdose, but equally don't stint on the length of medication, with the thought of reducing stress, when you're medicating sensitive fish. Here's my most important advice: Continue the salt bath or medication three days after you can find no Ich encysted on any fish, not even the least bit of maybe a possible one. You want to eliminate every encysted tomont and especially that last stray tomite swarmer. It only takes one successfully lodged in a gill to start the whole Ich cycle again and convince you that Ich is always "lurking" in the water.
Afterwards do several good big water changes, and then put fresh carbon in the filter to adsorb any residual drugs.
Auxiliary weapons. You will soon notice — from following the links just below — that there are several supplementary weapons against this parasite, none of which is utterly effective all on its own. They include: micron filtering, or filtering through diatomaceous earth, which traps many — but not all — the free-swimming tomites. The filtration would be more effective if all the water in the tank were passed through the filter before any of it were returned, but of course that's out of the question in our re-circulating systems. Do you see that some tomites would always remain in the water if you really depended solely on micron filtering to eliminate the swarmers of Ich?
Water changes that meticulously remove detritus do lower the level of organics in the aquarium, and heat does speed up the metabolism of the organism, thus forcing it more rapidly through its one vulnerable stage. Higher heat (above 86°) may stress the parasite.
And finally, during the malachite green and formalin medication, darkness helps, because sunlight oxidizes malachite green, making it less effective. I'm not convinced that our aquarium lighting is sufficiently intense to have any serious oxidizing effect. A 1997 article in Tropical Fish Hobbyist seemed to have started a brief-lived cult of attempting to cure Ich with complete darkness alone. Not sensible, in my opinion, since the creature is light-independent and doesn't even have an eye-spot.
Ich on the Web. A good condensed description of Ich's three-stage life cycle helps you know why you're "supposed" to do thus-and-so to get rid of it: supplement my description with Dr Erik Johnson's at "KoiVet". Though that is a pond and koi site, Dr Johnson's discussion of fish parasites applies to indoor tropical freshwater aquaria. When he says "asymptomatic carriers can sustain a population of Ich in a tank or pond for an indefinite period," remember, he means "unspecified" period, not "infinite" period. Dr. Johnson gives a good account of Ich treatment with salt in koi ponds.
Doug Thamm's often-copied article, "Way more than you ever wanted to know about Ich" of 1994 makes easy reading. It's as complete as it promises to be. Doug's recommendations aren't invariably the standard ones: medicate at the recommended dose, he tells us, but repeat it every 3 or 4 days, for a total of four treatments over 12 to 16 days, with a 50% water change before each remedication.
Dr.Harry Dickerson's Ich Page at the University of Georgia's College of Veterinary Medicine is very brief on the life cycle of Ich, but now I can't find what were the web's best electron micrographs of Ich in its various stages that formerly accompanied it. In 2009, dr Dickerson and his team of researchers at U of G College of Veterinary Medicine identified two new species of bacteria living symbiotically inside Ichthyophthirius and apparently essential to it.
Shawn Prescott's account of Ichthyophthirius is part of his "Diseases in Fish" series. Right after giving a good account of the three-stage life cycle of this pesky ciliate, however, he was a little too casual in referring to an unspecified "latent" stage, when I feel he might have done better by referring to "subclinical" or "asymptomatic" carriers, or to low temperatures slowing the cycle. But the color photomicrographs are good.
The University of Florida's Institute of Food and Agricultural Sciences weighs in with a U.S.D.A.-sponsored article co-authored by three Cooperative Extension agents, because Ich is a major plague for Florida's commercial ornamental and food fish farmers. So there's good Ich material at the University of Florida/IFAS site, "Extension Circular 920: "Ichthyophthirius multifiliis White Spot Infection of Fish." The document "Introduction to Freshwater Fish Parasites" is excellent in itself; it describes in detail Ich's appearance (with color pix), its life cycle and methods of prevention and treatments that reflect a fish-farming rather than an aquarium perspective: for example, malachite green remains too long in fish tissues for it to be used with food fish, like Channel Catfish, and it's too toxic in powder form to be used in outdoor ponds. And though the unstable reactions of copper sulfate in unbuffered, soft water like mine aren't a considerable issue when dealing with Florida's typically alkaline water, they might cause future problems for other fishkeepers with soft water and flexible pH. Treatments using potassium permanganate and sodium chloride are also described. From three to seven repeat treatments are recommended, to catch each generation of swarmers.
Ich myth. ...now you have no excuse for imagining that Ich has a dry spore stage that gets blown through the air and settles on the water, or that it can be transported in an aerosol mist or on a net that has air-dried overnight: "its spores are everywhere. in your tank. on your hands" writes a respected and usually knowledgable aquarist ...but you have no excuse for fearing that Ich is always lurking in your tank water, or even in your drinking water, or that it lies low in the gravel, dormant but just waiting for a cold spell to burst into action, etc etc... all Beemer: Bogus Misinformation Endlessly Repeated. Most Beemer isn't pure invention, but is based on some misinterpretation or mis-hearing of fact. If you keep a steady grip on the facts of Ichthyophthirius life stages, you may even be able to disentangle the particular misunderstanding that's at the root of each particular Ich myth. At the very least, please keep comparing stuff that you will be hearing with what you know. ...and don't just take the word of the Skeptical Aquarist!
The "dormant Ich" myth. In 1965, Dr. H. Reichenbach-Klinke could write, in the English translation of Krankheiten der Aquarienfische, (Stuttgart 1957), "We know as yet nothing about possible sexual reproduction in this species, nor does it seem to produce resting forms of any kind. Deviations from the described cycle may occur... young parasites may divide while on the fish, or mature forms, swimming free in the water, may produce swarmers without having been encysted." Dr. Herbert R. Axelrod bought the rights to the English translation of Reichenbach-Klinke's book and distributed an unre-edited photo reproduction of the text through his publishing company, TFH. But some years later, when he had a chance to oversee the translation and take a more active hand in editing Dieter Untergasser's Handbook of Fish Diseases (distributed by TFH, 1989), he added a figure (fig. 83) captioned "the vicious Ich cycle." This figure purports to show an alternate cycle for Ichthyophthirius multifiliis : "G. The [daughter cells] leave fish which have died. H. Two to three days later mature parasites mate (conjugation) for sexual reproduction. I. They produce cysts which can live for several weeks in the aquarium without finding a host." What's up with this? The figure is one of two in the book that are drawn by a different, coarser hand than the rest of the illustrations. They seem to have been added to the illustrations originally made for Untergasser's book.
The inserted caption I've quoted intrudes a note of spurious science that is not reflected in the text. You see, it's possible to be so sure of some conception that one is willing, perhaps only semi-consciously, to "bend the facts" a little, to fit the pre-conceived idea. The result is a "pious fraud," defended by high intentions.
It's possible that the myth of Ich "lying low" in the aquarium in an imagined "dormant" stage, may have come from confusing Ichthyophthirius multifiliis with a similar marine ciliate parasite called Cryptocaryon irritans. People like to call Cryptocaryon "Ich's marine counterpart." In marine aquaria, I'm told, Cryptocaryon (which means "hidden spore") can remain infective for up to thirty days, especially at low temperatures! If this is true, it's an insidious parasite, and much more difficult to eliminate than our familiar freshwater ciliate. But perhaps a habit of confusing the two — by calling them each other's "counterparts" — has helped create the myth of a counterpart "dormant" life stage for Ichthyophthirius multifiliis. Imagine a comparable situation among some more familiar organisms, like mammals. If you called sea otters the marine "counterparts" of the giant Amazonian river otters, then you might expect to find that the river otters would lie on their backs in the river water, cracking shellfish on stones which they carry on their bellies. But they don't; and they don't eat sea urchins like their marine "counterparts" either. "Counterpart," you see, is not a dependable category in the real world of animals. Not that it doesn't have a long history. Medieval thinkers expanded on some hints that they read in Roman natural history writers to imagine that every terrestrial creature had its "counterpart" in the sea. Part of the symmetry that made Creation perfect... But that's getting a long way from the fish tank...
At a forum recently I overheard
The Skeptical Aquarist's viewpoint is based on the idea that there are two possibilities: Ich is in the aquarium and killing the fish, or it is not in the aquarium. There are no other possibilities in his worldview. Based on that worldview, then there either wouldn't be ich in the world, or there wouldn't be fish in waterways infected with ich, because they would have been killed by the ich.
The epizootic that rages through fish in the confined waters of an aquarium finds itself so dispersed in natural waters, that only one of the swarm released from the cyst need find a host in order to carry on the organism's genes successfully. In comparison, the aquarium is an unnaturally confined situation, multiplying the parasite's chances. If someone is in the infectious stage of a highly contagious disease, compare your chances of catching it while walking at the same time in Central Park or while being confined with them in a subway car. In a parallel to your aquarium's isolation, scientists confined together to overwinter in Antarctica find that, once the station closes down for the winter and whatever colds they are carrying have run their course, no one catches a cold through the remainder of their isolation. Similarly, isolation protects your healthy aquarium.
Published misinformation. We all tend to repeat what we keep hearing and copy what we've read, without subjecting the information to skeptical review. Plenty of bold, authoritative-sounding BMER gets published. I can't tell you how much more authoritative this section on Ich looks, presented here in this format, than it did in my written notes...
In summer 2000, a (nameless) widely-read aquarium magazine printed an article so dis-spiritingly packed with Beemer concerning Ich, all in all so carelessly promoting such a recipe for a reader's future endlessly soured by cycle after cycle of Ich epidemics, that I just have to take it apart, for your sake. (By the time you've scrolled down this far, I figure you've followed the links to some dependable accounts, and you're well clued-up on the life-style of Ichthyophthirius multifiliis.)
Selection of fish. In the article I'm looking at, first you're advised, when buying fish, to "select only those that appear healthy... Examine the tank for dead fish and avoid purchasing from such tanks, unless you are confidant of the condition of the live specimens." Confidant? Fools rush in where angels fear to tread! Don't ever imagine you're buying a "healthy" fish from a tank that has any infected fishes, let alone dead ones.
Quarantine. This writer undercuts the very idea of quarantine by telling you that "for years I added fish purchased at local fish shops directly to my tanks. 99% of the time these transfers resulted in no outbreaks of Ich..." The author describes this "optional" quarantine as "a period of one to two weeks." Two weeks' time is just not long enough for newly-settled Ich to become visible.
Preventive intervention. "Some fishkeepers treat the quarantine tank with a low dosage of medication as a preventative measure." Preventive intervention is an excellent idea. Anyone who keeps turtles, snakes and lizards just takes it for granted that new purchases are harboring parasites, and they medicate to eliminate them. I think fishkeepers might do well to follow their example. Preventive intervention is especially recommendable if the fish are wild-caught. But a "low dosage" is a soft-headed idea. What is this medication for? Is this to be a fully effective treatment, or a magic gesture done just for good luck? Low dosage of any medication tends to spare precisely those organisms most resistant to the medication. It's the "low dosage" aspect that can make preventive intervention appear useless.
"Parasites can exist in a healthy tank and not show their presence until a fish is weakened and susceptible." This is almost true. The fact is that Ich ciliates can exist on a healthy-appearing host individual, and not show their presence, even for many weeks at cooler temperatures, or for several inconspicuous life-cycles on the gills of a resistant host. But Ich has no long-term "dormant" stage at large in the aquarium. Undaunted, this writer goes on: "Outbreaks of Ich, blamed on the addition of new animals with an alleged unseen condition, may be incorrect (sic). The reason for an outbreak may be due to another factor."
The writer goes on to imply that "an overstressed biological system" and crowding can result in elevated levels of toxic metabolites, increased stress and weakened fish. This is the "don't go out without your rubbers or you'll catch your death of a chill" school of epidemiology. I'm sure you understand that Ichthyophthirius is an organism. It is the vector of this disease. Stressed fishes may indeed be more vulnerable to many parasites, but if I. multifiliis is not present, they can't come down with "Ich." Not understanding this, the author of the article expends great care on equalizing temperatures when transfering fish to the tank, caution that might have been better spent performing a quarantine. Let me offer a parallel: Malaria is not the result of a swampy miasma of mal aria or "bad air;" it has a vector, and that vector is a blood parasite carried by a mosquito. Carefully closing the window shutters at night to keep out the mal aria might be effective — or quite in vain. It all depends, not on the nighttime mal aria, but on whether there is an infective mosquito already in the room.
Then for treatment I'm reading this: "Simply transfer the infected fish to the hospital tank and begin treatment... thus not exposing healthy specimens or live plants to unnecessary treatment and stress." Ai, ai ai! That means that only the fishes with big white spots get treated. The others remain to infect one another — until they develop visible white spots too, presumably. Do you see how the impression can get established that Ich is somehow everpresent, lurking dormant in the aquarium?
There's worse yet: "in a pinch" you are directed to place the diseased fish in a plastic hang-on-the tank filter-box floated in the main tank. Oh no! "The box can then be medicated without exposing the whole tank." You'd never do this, I'm sure. You know how important it is to avoid cross-contamination among a show aquarium, a Q-tank and a hospital tank. By now you also realise that the "whole tank" is already exposed to the parasite and must be considered to be in quarantine, at the very least.
This author also approves the addition of a tablespoon of salt per gallon, harmless enough, but certainly not enough to eliminate Ich. "Increasing the water temperature to 84o to 89oF has been noted to be an effective treatment," says this writer, but I hope you wouldn't count on that as sufficient just on its own.
Diatomaceous earth filtration and UV sterilizing techniques both get approving passing references in this article, as if they could eliminate Ich in a recirculating system.
This was an egregious example of Bogus Misinformation Endlessly Repeated, but be on guard for little snippets of Beemer in otherwise dependable contexts. For instance, an aquarist whom I admire recently advised a fishkeeper who had a tank where two goldfish had died of Ich, but which had run a month without any fish in it, "If the disease is still present, the parasite can bloom when the new fish are added." Nevertheless, his advice began well: "Don't chance a reinfection by simply putting more fish in the tank," but then, instead of recommending a quarantine, he recommended disinfecting the fish-free tank by running a regime of anti-parasite medication in it.
Acquired resistance to Ich. When Ich strikes, often there will be some fishes in the community that won't be affected. Recently, research at Cornell is confirming the long-held hunch of many aquarists that, once an individual fish has been cured of Ich, it has acquired some level of immunity.