Marine Ich, an infestation of Cryptocaryon irritans, is one of the two most common afflictions of saltwater fish; the other being Marine Velvet caused by Amyloodinium sp. (Michael, 2002 and Joshi, 2003). In this two-part series, I will explain some of the biology of this ciliated parasite and discuss the multitude of treatments, remedies, cures, and elixirs that have been put forth to save your fish and sometimes empty your wallet. My hope is to help you wade through the copious and often erroneous anecdote to find an effective treatment that best suits your particular needs and that of your fishes.

Before you can decide on a treatment, you need to be fairly certain what is the infectious agent of the fish. Some of the signs of infection with Cryptocaryon irritans are rubbing or scratching against decorations or substrate (this behavior is also known as glancing or flashing), breathing problems, an increased mucous layer, loss of appetite, abnormal swimming behavior, frayed fins, cloudy eyes, and, of course, the telltale white spots. These characteristic spots are usually described as appearing like small grains of salt stuck to the body of the fish. Even in the presence of all of these external signs, the best diagnostic tool is microscopic examination of fresh fin or gill clippings or skin scrapings. But, realistically, an extremely small number of us are ever going to perform this kind of differential diagnosis, myself included.

This disease is usually associated with several environmental triggers. Changes in water temperature, exposure to high levels of ammonia, nitrite, or nitrate, low pH levels, low dissolved oxygen, and overcrowding are all factors contributing to the onset of the disease. You could lump all of these in a general category of stress, but I find it more appropriate to think of all of these as wholly unnatural conditions. In fact, Cryptocaryon irritans is rare in the wild, and even more unlikely to be lethal (Bunkley-Williams & Williams, 1994). Ich is truly a disease that exploits the conditions of captivity to reproduce and easily find suitable hosts.

Host Susceptibility:

Cryptocaryon irritans has demonstrated a very low level of host specificity, meaning it will infect just about any teleost fish in a tropical marine environment. Cartilaginous fishes (sharks and rays) appear resistant, but everything else is susceptible to infection (Colorni & Burgess, 1997). It has even been proven to infect various species of freshwater fish that were acclimated to saltwater, as well as temperate marine fish that were kept at the upper limit of their thermal range (Yoshinaga & Dickerson, 1994; Burgess & Matthews, 1995).

Even though they are all possible hosts, experience has shown that there are definitely certain fish groups with higher and lower degrees of susceptibility. At one end of the spectrum are the eels that have shown a general resistance to Cryptocaryon irritans. On the opposite side are the surgeonfishes, with the Blue Regal/Hippo Tang (Paracanthurus hepatus) the "crowned king of Ich." I have dealt with literally hundreds of these fish, and I could probably count on one hand the number of Blue Regal Tangs that appeared to be completely free of infection. I would also place the cowfish, boxfish, and pufferfish fairly high on the susceptibility list. Generally, everything else falls somewhere in the middle.


The lifecycle of the parasite is interesting and important to understand when evaluating a treatment. The stage where the parasite is attached to a fish is called a trophont. The trophont will spend three to seven days (depending on temperature) feeding on the fish. After that, the trophont leaves the fish and becomes what is called a protomont. This protomont travels to the substrate and begins to crawl around for usually two to eight hours, but it could go for as long as eighteen hours after it leaves it's fish host. Once the protomont attaches to a surface, it begins to encyst and is now called a tomont. Division inside the cyst into hundreds of daughter parasites, called tomites, begins shortly thereafter. This noninfectious stage can last anywhere from three to twenty-eight days. During this extended period, the parasite cyst is lying in wait for a host. After this period, the tomites hatch and begin swimming around, looking for a fish host. At this point, they are called theronts, and they must find a host within twenty-four hours or die. They prefer to seek out the skin and gill tissue, then transform into trophonts, and begin the process all over again (Colorni & Burgess, 1997).

Many hobbyists are fooled into believing they have cured their fish of the parasites, only to find Ich present again on fish a few weeks later; a reason why following through with a full treatment protocol is so important. Don't make this mistake and be lulled into a false sense of security. The parasites may be in a stage where they are merely regrouping and multiplying for their "next offensive." In the wild, this sort of massive reproductive phase ensures that a few will find a suitable host to continue on the cycle. In the close confines of our aquariums, though, it means comparatively massive infection rates.

There is another interesting observation I found in my investigations concerning the biology of Cryptocaryon irritans. Mature trophonts leave the host and tomites exit the theront/cyst in the dark (Yoshinaga & Dickerson, 1994). Imagine if you will, a fish that randomly acquires a single Ich parasite. After a couple of days when the trophont is well fed, it prepares to drop off its host but waits for the environmental trigger of darkness. Meanwhile, the fish prepares to "bed down" in its favorite hiding spot in the aquarium; the same fish occupy the same spot practically every night. Now, the trophont leaves the fish, encysts, and begins to multiply. Several days to weeks go by and that same fish returns to its same spot at night, only this time there are hundreds of infectious theronts seeking out a host/victim in the same area. I am sure some of you are thinking that this is absolutely diabolical. Others can appreciate the simple beauty of this plan. To me, it is just another reminder of how remarkable evolution and adaptation is.

Given all of this "planning," it seemed strange to me to read that not all the theronts will be able to find a host. Actually, given ideal laboratory conditions, only between five and twenty percent succeed, but that still adds up to an awful lot of parasites. Given this kind of infection rate and the rate of reproduction, the total number of parasites can increase approximately ten times every week (Colorni & Burgess, 1997).

Concerning taxonomy, Cryptocaryon is currently still a monotypic genus (meaning there is only one species in this genus). Although, there is research to suggest that there may actually be several distinct species. There are at least various isolates from different geographic regions; even if their differences are not substantial enough to warrant designating separate species (Colorni & Burgess, 1997). I, and several of my friends in the business, can testify that there are marked increases in Cryptocaryon irritans outbreaks and mortalities when mixing fish from the Caribbean with those of the Indo-Pacific. It is possible that this higher incidence in problems could be the result of fish that have evolved a limited immunity against their native variety coming into contact with an unfamiliar strain of parasite.

Preventative Medicine:

The best course of treatment is prevention. All new fish should be quarantined for at least one full month. This helps ensure that the fish are healthy, but it also gives them time to get over any shipping trauma, to get used to a new diet, and to put on weight after withstanding often insubstantial feedings at retailers, wholesalers, and collecting stations. Best of all, this will occur in a competition-free environment.

I have found the best quarantine/hospital tanks to be bare bottomed (no crushed coral or sand) and decorated with inert, nonporous, and "easy-to-clean-and-sanitize" items. Short sections of various diameter PVC pipe work very well for shelter. Live rock does not meet these criteria and therefore I do not recommended its use. It is best to not use any calcareous materials as they will absorb and interfere with some medications.

There is also another possible benefit to using all of these smooth, artificial materials in your quarantine tank. In studying outbreaks of Cryptocaryon irritans in Brown Spotted Grouper (Epinephelus tauvina) at an aquaculture station, Rasheed (1989) found that fish kept in concrete vessels routinely fell victim to Ich while those kept at the same facility with identical care, but in fiberglass containers suffered absolutely no infestations. She theorized that the cyst stage of the parasite found the smooth sides of the fiberglass tanks inhospitable. While not proven, it is very interesting and definitely something to keep in mind. At the very least, this type of setup is extremely easy to clean and disinfect if necessary.

I prefer to filter the tank with sponge filters. I usually have one running but tucked away in the sump of my display tank. This way, I can keep the quarantine tank empty and packed away in the garage when it is not needed. Some people may be concerned about the sponge filter acting as a so-called "nitrate factory", but the amount of nitrate produced from a comparatively small sponge filter should be negligible. When I do need to use the quarantine tank, I merely drain some water out of the display tank into the quarantine tank, add the sponge filter and a heater (I generally target 80°- 82°F to speed up the life cycle of any parasites), and it is ready. You may also want to include a powerhead into the quarantine setup for fish that require brisk water movement. The entire setup process should take less than an hour and save the electricity and space of maintaining the quarantine tank continuously. Also, keeping the quarantine tank empty spares the temptation of turning it into a full-blown reef or fish tank!

If you do not have a sump on your display tank, a hang-on type filter, such as those with "bio-wheels," work well. Keep the bio-wheel running on the display tank and move it to the quarantine tank when needed. The only precaution is to remove the activated carbon filter cartridge when using any medications.

It is my strong preference and my general recommendation to never add any medications to a display tank. In my experience, it is always better to remove all the affected fish to a separate quarantine/hospital tank for treatment. This ensures that none of the display tank's other inhabitants such as corals, bacteria, worms, amphipods, copepods, or mysid shrimp are affected. Also, if you keep the fish in quarantine for one month without infection, you can be sure that any Ich parasites and their eggs have hatched and died without a host. Note that Cryptocaryon irritans requires a fish host. They cannot complete their life cycle with the rock, sand, or any invertebrates.

Some people draw a distinction between quarantine tanks and hospital tanks, with hospital tanks being designed like I have described above, and quarantine tanks much more like displays lacking other fish. I don't draw this distinction and choose to quarantine all my fish using the means described above. The theory is that fish kept in a more natural setting will be under less stress, and therefore more likely to resist disease. In the event a fish does come down with a disease, it is transferred to a bare-bottom tank for treatment. While this may have some merit, I cannot be bothered running two tanks just for temporarily holding fish.

Treatment Option 1 - Copper:

Copper is a highly effective medication against Cryptocaryon irritans when dosed and maintained in the proper concentration. The references I found varied in their recommended dosage:

Andrews et al, 1988: 0.15-0.30 mg/l
Bassleer, 1996: 0.25-0.30 mg/l
Gratzek et al, 1992: 0.115-0.18 mg/l
Noga, 2000: 0.15-0.20 mg/l
Untergasser, 1989: 0.15-0.20 mg/l*
                  *(recommends to be used with Methylene Blue)

I am going to abbreviate my advice and simply suggest to: "Always follow the directions of the manufacturer of whichever brand of copper medication you employ, and always use a test kit to verify the dosages." Copper has a narrow range of effectiveness and levels must be monitored at least daily.

Copper has several disadvantages in treating Ich. First, at too low a dosage, it is ineffective. Secondly, at too high a dosage, it could kill all your fish. Daily, or better yet twice daily, testing is required to maintain an appropriate and consistent level of copper. Even when within the appropriate ranges, some fish cannot tolerate copper. Some of the fish more sensitive to copper are lionfish, pufferfish, mandarins, blennies, and any other scaleless fish. Copper is also a known immunosuppressive, making fish more susceptible to secondary infections. Invertebrates are extremely sensitive to copper and cannot be housed in a tank undergoing this treatment. Lastly, copper cannot be used in the presence of any calcareous media. Live rock, sand, crushed coral, and dead coral skeletons will all adsorb copper, rendering it useless a treatment.

Copper specifically targets the infectious, free-swimming theront stage of this disease, as being buried deep in the skin of the host protects the trophonts; the cyst walls of the tomonts are similarly impervious (Colorni & Burgess, 1997). Knowing this and the life cycle of Cryptocaryon irritans, monitoring and dosing as needed in the evening right before the lights go out is going to be the most effective method. This should ensure optimal treatment concentrations at the most beneficial time.

Copper is probably the most popular method of treating Cryptocaryon irritans, but is not my first choice. It is far too labor intensive for me to recommend to the general public, has a large risk of overdose, lowers the fish's resistance to other diseases, and can cause serious damage to the kidney, liver, and beneficial intestinal flora of the fish being treated. Damage to intestinal flora is what many hobbyists point to as a possible contributing cause for Head and Lateral Line Erosion (HLLE), although there is currently no definitive cause of HLLE.

Treatment Option 2 - Formalin:

Formalin can be administered one of two ways; either in short dips with saltwater or used continually in a hospital tank. The dosage for the continuous use is 1 ml of the 37% stock solution for every 25 gallons of quarantine tank water (Bassleer, 1996). I prefer the formalin dip to continuous use because formalin is a fairly toxic compound. Also, with no commercially available test kits to monitor the concentration, it would be difficult to dose an entire tank and account for evaporation, absorption, etc.

To prepare the dip, I take 5 gallons of tank water and add to it 3.75 ml of 37% formalin. I also aerate the water vigorously to ensure there is maximum dissolved oxygen. The dip should last 30 to 60 minutes. As when using any medication, it is best to monitor the fish's reaction and be prepared to act if it appears in distress. When the dip is complete, net the fish, place it back into the hospital tank, and discard the dip water. This protocol should be repeated every other day for two weeks.

I would like to remind readers of a few precautions regarding the use of formalin. First, it is a carcinogen. Formalin is an aqueous solution of carcinogenic formaldehyde gas, so gloves should be worn and the area should be well ventilated when using it. Secondly, formalin should not be used if fish have open sores, wounds, or lesions. It is likely to cause tissue damage to these open wounds. And lastly, formalin can rob the water of dissolved oxygen. That is why proper aeration is so crucial. For that reason, do not use formalin if the water temperature is 82*F or higher (Noga, 2000 and Michael, 2002).

Treatment Option 3 - Copper & Formalin:

It is possible and sometimes preferable, like in the case of heavy infestations of Cryptocaryon irritans and Amyloodinium sp., to use copper in conjunction with Formalin in a quarantine/hospital tank. The same warnings about sensitive fish still apply. If a fish is sensitive to either copper or Formalin, they are not safely exposed to the combined protocol. At this point, it is "cure or kill." You will either cure your fish or kill it from poisoning. It is the most aggressive and dangerous treatment described in this article.

Treatment Option 4 - Hyposalinity:

Low salinity has been demonstrated to be an effective treatment against Cryptocaryon irritans (Noga, 2000). A salt level of 16 ppt or approximately 1.009-1.010 specific gravity at 78-80*F for 14 days was reported to kill the parasite. I have never experienced problems when placing fish into a hyposalinity treatment, but have routinely witnessed fish showing obvious signs of distress when brought back to normal salinity levels too quickly. For that reason, I try to limit the specific gravity increase 0.001-0.002 points per day.

One of the alleged benefits of this treatment is the resulting conservation of energy for the affected fish. Reef fish have to constantly drink saltwater and excrete the salt to maintain the proper osmotic balance. Lowering the salinity of the surrounding environment eases this energy demand on the sick fish, thereby allowing them to expend more energy towards fighting the infection (Kollman, 1998 and Bartelme, 2001). On the contrary, keeping fish in low salinity means that they don't "flush" their kidneys sufficiently. After long-term exposure, this can cause kidney failure and kill the fish (Shimek, pers. comm..)

The drawbacks to this treatment are the same as for many of the treatment options discussed above. Invertebrates and certain fish will not be able to tolerate it, so you should not apply a hyposalinity treatment in a display tank. Sharks and rays are two fish groups that do not tolerate this procedure. I would also not recommend this approach in the presence of live rock or live sand. The hyposalinity treatment will likely kill the worms, crustaceans, mollusks, and other life in and on the substrate, causing a severe drop in overall water quality.

I have another word of caution when using this treatment. I would strongly suggest the use of a refractometer or perhaps a salinity monitor. Swing arm style box hydrometers are notoriously inaccurate. The glass, floating style hydrometers are better, but easily broken. An accurate measure of the salinity could mean the difference between being inside the effective treatment range or being too high and ineffective or too low and jeopardizing your fish.

Even given its few drawbacks, hyposalinity is a great method of curing infected fish of ich in a proper hospital tank. Of the treatment options discussed this far, in my opinion, it is by far the safest. While none of these options is appropriate for use in a display tank, and all have their drawbacks, weighing the pros and cons of each leads me to recommend hyposalinity above the others.

Treatment Option 5 - Daily Water Changes:

John Walsh related this method in a presentation given to the Pittsburgh Marine Aquarium Society, Inc. It is safe and effective for all marine fish (Colorni, 1985) and is my preferred first course of action. Fish are put into a quarantine/hospital tank and then everyday for two weeks the tank is completely cleaned and a 50% water change is performed. While the size of this water change may concern some aquarists who are not accustomed to water changes of this magnitude, as long as you are careful about matching the temperature and salinity, you should not experience any problems. This method helps to remove the tomites, tomonts, and theronts from the tank and lessens the chance of reinfection. The fish should remain in quarantine for an additional month to ensure the treatment has worked and to allow them time to gain strength.

This method is best used as a preventative when a fish is first acquired. It is also useful for mild infestations or when other more aggressive treatments cannot be used due to species sensitivity. The best thing about this kind of treatment is it is safe for all fishes and invertebrates. One of the other benefits is the daily water changes should help you maintain optimum water quality and therefore should stimulate the fish's immune system to combat any secondary bacterial infections that might be attacking the vulnerable areas where the Cryptocaryon irritans parasites have burrowed into the skin. This is in contrast to copper or Formalin, which are both immunosuppressive, and may actually promote secondary infection.

Variations of this method (and the likely source for the original idea for the treatment) have been suggested and used successfully by Colorni (Colorni, 1987 and Colorni & Burgess, 1997). They involve moving the infected fish between two tanks with the tanks being cleaned and dried in between uses or removing a sand substrate and replacing with new sand every three days. I don't like the idea of handling a sick fish that much using the tank transfer method. While fishnets are designed to be soft and supple, it can still be dragged across the fish's eye. I have found the more you have to manipulate a fish, the more likely it is to contract a secondary bacterial infection like pop-eye or cloudy eyes. The substrate removal method is interesting. The sand is supposed to be an ideal media for attracting the encysting parasites. Removing the sand every three days removes the tomonts with it. Utilizing aragonite sand for this purpose is expensive (unless you happen to live somewhere that Southdown sand is readily available) and very messy. Silica sand is widely available, cheap, and will create slightly less cloudy water, but it is still not as clean and easy as the water change method. I have found the water change protocol to be just as effective and considerably more practical than either of Colorni's methods.

Hopefully, I have provided information about some of the biological characteristics of this parasite and clear instructions on the most common cures. In the second part of this series, I hope to discuss some of the newer and/or more experimental treatments that have been proposed or recently appeared in the aquarium marketplace. Please keep in mind the life cycle and accepted cures for this disease in the context of discussing the second group of treatments in part two of this series. In many cases it is difficult to separate an allegedly observed "cure" from simple variations in individuals' natural immunity.


I would like to thank Eric Borneman, Adam Cesnales, moonpod, and Dr. Ron Shimek for their editorial content and ideas. Their suggestions were quite helpful in crafting this two-part series.

Link to Part II

If you have any questions about this article, please visit my author forum on Reef Central.


Andrews, Dr. Chris, Adrian Exell, and Dr. Neville Carrington. 1988. The Manual of Fish Health. Blacksburg, VA: Tetra Press. pages 166-168 and 190-195.

Bartelme, Terry D. 2001. "Treating Saltwater Ich Without Medication" Tropical Fish Hobbyist, January 2001.

Bassleer, Gerald. 1996. Diseases in Marine Aquarium Fish: Causes, Symptoms, Treatment. Westmeerbeek, Belgium: Bassleer Biofish. pages 51-52.

Bunkley-Williams, Lucy & Ernest H. Williams, Jr. 1994. "Disease caused by Trichodina spheroidesi and Cryptocaryon irritans (Ciliophora) in wild coral reef fishes" Journal of Aquatic Animal Health 6:360-361, 1994.

Burgess, P. J. & R. A. Matthews. 1995. "Fish host range of seven isolates of Cryptocaryon irritans (Ciliophora)" Journal of Fish Biology 46:727-729, 1995.

Calfo, Anthony pers. comm.

Colorni, Angelo. 1985 "Aspects of the biology of Cryptocaryon irritans, and hyposalinity as a control measure in cultured gilt-head sea bream Sparus aurata" Diseases of Aquatic Organisms, 1:19-22, 1985.

Colorni, Angelo. 1987. "Biology of Cryptocaryon irritans and strategies for its control" Aquaculture 67:236-237.

Colorni, Angelo & Peter Burgess. 1997. "Cryptocaryon irritans Brown 1951, the cause of 'white spot disease' in marine fish: an update" Aquarium Sciences and Conservation 1:217-238, 1997.

Dixon, Dr. Beverly A. pers. comm.

Fenner, Robert pers. comm.

Gratzek, Dr. John B., Dr. Richard E. Wolke, Dr. Emmett B. Shotts Jr., Dr. Donald Dawe, and George C. Blasiola. 1992. Aquariology: Fish Diseases & Water Chemistry. Blacksburg, VA: Tetra Press. pages 105-107.

Joshi, Sanjay. 2003. "Top 5 Marine Fish Parasites" Aquarium Fish Magazine, September 2003.

Kollman, Rand. 1998. "Low Salinity as Quarantine and Treatment of Marine Fish Parasites" SeaScope, Volume 15, Spring 1998.

Michael, Scott. 2002. "Fighting Marine Parasites" Aquarium Fish Magazine, October 2002.

Noga, Edward J. 2000. Fish Disease: Diagnosis and Treatment. Ames, IA: Iowa State University Press. pages 97-99, 282-283, and 286-288.

Rasheed, Victoria M. 1989 "Disease of Cultured Brown-Spotted Grouper Epinephelus tauvina and Silvery Black Porgy Acanthopagrus curvieri in Kuwait" Journal of Aquatic Animal Health 1:102-107, 1989.

Untergasser, Dieter. 1989. Handbook of Fish Diseases. Neptune, NJ: TFH Publications. pages 95 and 127.

Walsh, John pers.comm.

Yoshinaga, T. & H.W. Dickerson. 1994. "Laboratory Propagation of Cryptocaryon irritans on a Saltwater-Adapted Poecilia Hybrid, the Black Molly" Journal of Aquatic Animal Health 6:197-201, 1994.

Further Reading:

Bartelme, Terry D. 1999. “Formaldehyde: Friend or Foe?” SeaScope, Volume 16, Summer 1999.

Dickerson, Dr. Harry W. 1994. "Treatment of Cryptocaryon irritans in Aquaria" SeaScope, Volume 11, Summer 1994.

Fenner, Robert. 1998. The Conscientious Marine Aquarist. Shelburne, VT: Microcosm Ltd.

Fenner, Robert. 2003. "Cleaning Symbiosis Among Marine Fishes" Tropical Fish Hobbyist, February 2003.

Moe, Martin A. Jr. 1989. The Marine Aquarium Reference: Systems and Invertebrates. Plantation, Florida: Green Turtle Publications.

Siegel, Terry. 2002. “Cryptocaryon: An Old Enemy” Marine Fish & Reef 2002 Annual.

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Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro -