I hear that if you feed garlic, slap your mother, and then spit on your father that you can cure ich! So that was sarcasm.... Ok, so for those out there that think that Cryptocaryon irritans or "marine Ich" is always present in any system... you are completely wrong. This organism is an obligate parasite and must have a host to survive. The members that have listed the time spans of 6-8 weeks are absolutely right about quarantining anything that you are going to put into your display tank (Thank you all for not perpetuating myths). Every fish... coral... live rock... invertebrate (shells)... etc...
Why is it important to quarantine all of these things? Well lets consider the life cycle first as this is essential for knowing how to control this parasite. C. irritans has a direct life cycle that consists of multiple stages. The feeding trophont stage resides on the fish (parasitizing skin, gills, fins). The trophont stage is actually what you see as the characteristic pinpoint white spots. Now just because a fish doesn't present with white spots doesn't mean there isn't a subclinical infection at work. Remember that the gills are also a site of infection and one that is not easily visualized without taking a gill clip and looking at it under a microscope. So the trophont stage is a ciliated stage that feeds on the fish and eventually will drop off the fish (either at the death of the fish or as mucus is sloughed off). After leaving the fish the trophont will fall onto a surface (sand, rock, side of tank, marine plants, etc) and encyst into a stage that is referred to as the tomont. This stage is environmentally hardy and is resistent to therapeutic treatments. Inside the trophont the parasite begins to asexually reproduce and divide with the end product being hundreds of tomite stages. The encysted tomites within the tomont will eventually be released and develop into ciliated infective theront stages. These are the stages that actually infect the fish by penetration upon contact. The theront stage is also the only stage that is targeted for treatment with copper, formalin, etc. Ok, so this entire process can take about a week depending upon temperature. I won't get into that because it varies depending on strain, publication, etc.
OK, so back to quarantining and why it can ensure that you have an Ich free tank.
Well as far as quarantining fish that is a no brainer. You quarantine and treat fish in a separate hospital tank. Problem solved! You don't introduce the trophonts on the fish into the display tank. No establishment of the life cycle in the tank.
On to why you need to quarantine rocks, plants, corals, etc. As I detailed in the brief summary of the life cycle, the tomont stage will encyst on the surface that it lands on. So you come home after buying a nice piece of live rock that you happened to purchase from a tank that also had fish. One of these fish happened to be infected with C. irritans and you didn't notice. You think it's okay to go ahead and not quarantine this piece of rock. A few days to a week later you notice small white spots on your fish in your display tank. Guess who introduced the parasite! YOU! I want to clearly stress that C. irritans is an obligate parasite. It MUST have a fish host in order to persist and survive in your tank. By letting the tank remain fishless for 6-8 weeks you account for the development of the parasite to the theront stages which eventually use up their energy stores and die after a period of 24-72 hours when they fail to penetrate a fish. Stress doesn't help the situation, but stress alone is not the reason that the fish has C. irritans. It was introduced into the tank at some point.
This rant has been a long time coming and is not directed at any single user. I wish that aquarists would be responsible enough to search for reliable sources of information regarding anything they are interested in knowing, but especially the subject of infectious diseases of fish. And by reliable sources I mean not just reading what someone has posted on a forum (even my post here - go look up this information for yourself!- I believe a user posted some links to a few review articles, etc above) I have heard it all when it comes to curing "ich"! From the traditional garlic to anything else. In my belief and from what I have found in the scientific literature, the only sure way to have a display tank free of C. irritans is to quarantine all things that you put into the display tank. Treat fish that are infected.
As a graduate student working with parasites of fish, I have a working knowledge of how many fish parasite life cycles work (protozoan, myxozoan, digenean, etc) and would like to help prevent the spread of the myths associated with marine ich.
