Amongst the two most common "scourges" of marine reef fish diseases, the most common is saltwater ich or white-spot disease (the other being the dinoflagellate Amyloodinium or "velvet"). As far as biological causes of captive mortality go, "crypt" is the hands-down winner for sources of loss. All to regrettable, as this simple protozoan could be eliminated by simple pH-adjusted freshwater bath protocols in the course of collection from the wild, transport through chain of custody/supply, and otherwise can be largely avoided by dips, quarantine from hobbyists' and institutions' tanks.
Amongst the most myth-ridden subjects of marine aquarium keeping, "marine ich" must be near the top. Some folks (in print no less) have stated that the cause of this disease is bacterial, viral... directly environmental! Otherwise known as "Crypt", the causative organism of marine ich is a ciliate protozoan (single-celled animal) known to science as Cryptocaryon irritans (Brown 1951). This Ciliophoran was described by Dr. Eleanor Brown in 1951.
Direct observation of the responsible micro-organism is possible with any medium power microscope, the adult size being up to about 0.5 mm in diameter. Remove some body slime from an infested fish by skimming a microscope slide along its side from the direction of head to tail, and smear this onto the surface of another slide. You might improve contrast by staining the slide specimen with a drop or Methylene blue or malachite. Adult ich appears as a roundish blob with a larger four-lobed macronucleus. The outside of the cell covered by numerous small "hairs" (cilia).
Symptomology though not restricted to crypt infestations alone include telltale salt-like dots on the fish hosts bodies, fins and eyes, excess mucus production, flashing/scratching, hiding, lack of feeding, fast, labored to slow labored respiration near death, and cloudy eyes.
Cryptocaryon irritans is a parasite with a direct life cycle, i.e. requiring no intermediate host like an invertebrate to complete its life cycle. The time per generation is temperature dependent, ranging from a few days for tropical to a week or more. If one considers the possibility of "resting stages", marine ich can wait out weeks to months before seeking out fish hosts.
A) Starting with a stage feeding (call trophonts) on its fish host. These are embedded below the epithelium (upper living skin layers) of host fishes, under copious amounts of mucus, not affected by chemical treatments.
B) Protomont stage leaves the fish, drops to the bottom and forms a resting/developmental cyst (tomont) stage persisting for 3-30 days generally. Becomes attached to and transmissible by any wet object. For about a day at 78 degrees F. reproduction occurs by binary fission; that is, by each cell dividing into two, possibly producing two hundred individuals (then called theronts). These encysted stage individuals are not affected by chemical treatments.
C) After 3-7 days, as tomites or theronts they break out of the cyst (typically at night, when reef fishes are often "sitting on the bottom") and swim into the water in search of a host fish, in a to several hours to a day or two at elevated temperatures they must find a fish host or die. If the parasite is lucky (and its host fish not so) it will find a host and burrow into its skin or gills. This "free-living" swimming stage is the opportune moment for chemical treatment.
You'd think that with such a potential killer being so common that there would be a simple standard operating procedure for its treatment. Guess again. The state of development of the hobby and huge turnover of hobbyists (more than 100% per year) dictate that the non-scientific aspects of knowing (faith, intuition...) hold sway in allowing nonsense "remedies" to persist. Investigate your options thoroughly.
You'll ready to pre-pay for that "pound of cure" for sure once you've had an encounter with crypt. But I hope you will instead avoid having to treat your fishes at all for this external parasite by following simple quarantine practice. Know that there are some fish-groups like Surgeons and Rabbitfishes that are "ich magnets", much more susceptible to infestations, though virtually all marine fishes, including sharks and moray eels can become hosts given virulent exposure and/or impugned environmental circumstances. This being stated, the single best way for you to not have to deal with marine ich (or most all biological diseases of livestock) is to employ a few-week (2-3) isolation/quarantine regimen. This period of time will give your new fish (and non-fish) livestock a chance to "rest up", and show signs (if any) of disease development. Some folks advocate pre-emptive chemical treatment for saltwater ich, I don't. Better to do your best to acclimate new livestock, keep them separate and administer treatments only if definite signs of parasites are evidenced.
All diseases are to degrees environmentally mediated. That is, the physical, chemical and social make-up, foods/feeding and a myriad of other factors directly and indirectly dispose an organism to/from health to disease. Many systems teeter on being just about parasite free, though possessing latent infestations of parasites. With slight changes in water quality, nutrition or social interaction, this balance can be tipped either way.
After Observing Infestation:
Many products, chemicals have been advanced as being efficacious in treating for Crypt, some in combination with others. In general the more effective treatments are more potentially toxic... all are and their mis-use is likely more a source of mortality than the actual parasites they're being used to eliminate. Be aware that there are a few commercial "reef safe" remedies (pepper-sauce, garlic...) on the market that are unreliable to put it mildly. Rather than saving fish lives these persistent "cures" kill-off hobbyists by the droves. Avoid them by getting on the internet, converse with fellow hobbyists re what works and doesn't.
Temperature effects. As with freshwater ich, it's advised to raise your systems temperature to speed up the life cycle of Crypt while you're treating for it. If your livestock can handle it, increase your heating to the mid 80's F along with whatever other treatment regimen you employ.
A) Hyposalinity, lowered specific gravity. Some advocates place lower spg. as low as 1.009. This can work if your fishes are not too challenged already or the pathogen too virulent, however it will not effect a permanent system cure. Know that most common measures of specific gravity are temperature specific and that most non-fish livestock will not tolerate the lower limit (14-16 ppt salinity) necessary to kill off the parasites. Therefore your fishes will have to be separated from your non-fish livestock if you're using hyposalinity as a treatment mode. And there are exceptions, variations to consider using hyposalinity. Cartilaginous fishes (sharks, rays) cannot be treated in this fashion... and such osmotic changes need to be made gradually (over days).
B) Ionic copper solutions, chelated and not. Copper is an old-timey, but proven method of eliminating Cryptocaryon. Solutions come in two varieties, bound up with a "carrier" molecule (chelated) and "free" (as in copper sulfate solutions). Both types have their benefits and shortcomings. Chelated copper "lasts longer" in marine water, cutting down the frequency of administration, whereas free copper is more available, readily effective. Note that you need to have/use a test kit for either type of copper used and that they are different test kits. Whichever format of copper is utilized it should be tested for and if necessary added to twice or more often daily. Often testing, adjusting copper levels assures that a "physiological dose", sufficient concentration (0.15-0.25 ppm over 7-10 days plus) of cupric ion is present to kill the tomite/theront stages.
C) Metronidazole (aka Flagyl), Quinacrine Hydrochloride, Quinine Sulfate. Not effective consistently.
D) Formalin or formalin/malachite or formalin/copper mixtures. Can be useful for initial infestations, treating large numbers of specimens, but the biocide formalin is dangerously toxic in the hands of the uninitiated. If used, shy on the low concentration, utilize extra aeration/circulation and closely watch your fishes and biological filtration.
E) Various schemes at moving infected fishes to new quarantine/treatment tanks, and vacuuming either bare-bottomed or systems with substrates: These approaches hope to capitalize on timing to eliminate intermediate forms of Crypt. Can be successful, but requires timeliness, effort on your part.
The Real Deal: Treating Fishes in Isolation, Allowing the Main System to "Go Fallow"
There are no "reef-safe" and effective treatments for crypt. NONE. Curing infested fishes involves separating them from non-fish livestock and treating them in that other system (or alternatively moving the non-fish livestock). Infested systems can be made "crypt-free" or better "crypt-virulence-reduced" by having them kept free of fish hosts for several (4 or more) weeks without fishes. If practical, elevating temperatures and possibly lowering specific gravity (to the tolerance of other non-fish livestock) can be employed to "speed up" the loss of virulence of the parasites. In practical terms we are generally talking the low to mid 80's F. and 1.017-1.018... with these values adjusted over days time. Care must be exercised in not possibly transmitting disease organisms from the quarantine system... on nets, containers, hands... anything wet, and drying, otherwise sterilizing quarantine tanks and gear between uses.
About Ultraviolet Sterilizers:
UV's cannot effect an actual "cure", though they do seem to do so in some cases where slight improvement in the overall system water quality may tip the balance between health/disease in the fishes side. Otherwise UV use can significantly reduce the number of free-swimming tomites. An initially improved situation is often perceived in initial infestations with a synchronized population of adults cycling off their hosts (every 3 to 7 days)... only to resurface in great numbers due to the confines of captivity.
About "Crypt Free" Systems:
There are such things, but unless the aquarist is diligent in altogether excluding these parasites through quarantine, treatment outside their main displays, most aquariums will instead host latent infestations... with discernible populations of Cryptocaryon coming to be through environmental challenge/s to their fishes. In actual fact cysts of Cryptocaryon can stay viable for a few to several months, hence ultraviolet sterilization, use of biological cleaners, allowing systems to go fallow... only decreases the number and virulence of these parasites. Once in a system, the system itself is infested and the only practical means of control becomes providing an optimized and stable environment.
Pandemics of saltwater ich have waxed and waned during the entire history of the captive marine hobby. It is likely that these infestations account for a large percentage of hobbyist attrition. This is regrettable and avoidable by simple quarantine procedures and adherence to a reliable treatment protocol. Isolation of fish livestock, hyposalinity and elevated temperature, administration of copper medication with testing will cure all but the most entrenched cases.
http://www.wetwebmedia.com/mardisease.htm, The three sets of factors that determine livestock health
http://www.wetwebmedia.com/dips_baths.htm, Net et al. dips to prevent spreading communicable diseases
http://www.wetwebmedia.com/quaranti.htm, Quarantine procedures
http://www.wetwebmedia.com/AqBizSubWebIndex/ltrquartrdbiz.htm, Letter to the trade encouraging mass marine quarantine
Steven Pro's excellent ich articles that start here: http://reefkeeping.com/issues/2003-08/sp/index.htm & Terry Bartelme's http://www.advancedaquarist.com/issues/nov2003/mini1.htm http://www.advancedaquarist.com/issues/dec2003/mini2.htm http://www.advancedaquarist.com/issues/jan2004/mini3.htm http://www.advancedaquarist.com/issues/feb2004/mini4.htm http://www.advancedaquarist.com/issues/mar2004/mini5.htm
Barteline, Terry. 2001. Hyposalinity therapy. Treating saltwater ich without medication. TFH 1/01.
Barteline, Terry. 2001. Cryptocaryon irritans. An update on the scourge of marine aquariums, pt.s 1 & 2. FAMA 2,3/01.
Blasiola, George C. 1976. A review of "white spot", Cryptocaryon irritans. Marine Aquarist 7/76.
Burgess, Peter. 1995. Marine whitespot disease. A fresh look at a salty problem. FAMA 1/95.
Goldstein, Robert J. 1974. Cryptocaryon vs. formalin. Marine Aquarist 5:1, Jan./Feb. 74.
Herwig, Nelson. 1978. Treatment of Cryptocaryon- saltwater ich. TFH 2/78. (largely the same in FAMA 4/81)
Herwig, Nelson. 1979. Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Charles C. Thomas, Publisher. Springfield, IL.
Michael, Scott W. 1995. Attack of the ich. This problem is in the hands of the aquarist. AFM 7/95. same as below.
Michael, Scott W. 1998. Ich cure. If you keep your fish in a reef tank, prevention is really the only cure. AFM 1/98.
Michael, Scott W. 2001. Saltwater ich. AFM 8/2001.
Miller, Gar and Michelle Liu. 1990. Nonchemical control and eradication of Cryptocaryon irritans. FAMA 3/90.
Straughan, Robert P.L. 1960. Salt water "ick" deadliest marine killer. TFH 10/60.
Ulrich, Theresa. 2001. Dealing with cowfish and other "ich magnets". FAMA 8/01.