Here is part of an article by Trevor Jones, it seems to be well documented
Is "Ich" always present in our aquaria?
There is a widely held belief in the marine aquarium hobby that "Ich" is always present in our aquaria and this belief is often repeated on marine bulletin boards. There is much information in the scientific literature that contradicts this belief.
C. irritans is an obligate parasite (Burgess and Matthews, 1994; Dickerson and Dawe, 1995; Yoshinaga and Dickerson, 1994). Obligate means the parasite can not survive without infecting its host, in this case, fish. Theronts have been shown to die if a suitable host is not found within the required time. Yoshinaga and Dickerson (1994) found that few theronts (0.34%) were viable 12.5 hours after excystment and Burgess and Matthews (1994) found that no theronts were viable 18 hours after excystment. Colorni (1985) found that some excysted tomites (=theronts) were observed to be moving weekly after 48 hours. While the life span of the theronts appears variable, it is limited and all will die without finding a suitable host.
If an aquarium has no fish in it, and there are no additions of fish, or anything else that could be carrying trophonts, tomonts, tomites or theronts for a period of 6 weeks or longer, all parasites will have died. An aquarium such as this is an obvious exception to "Ich" always being present.
Many fish collected for marine aquariums will not be carrying "Ich". Incidence of C. irritans in wild fish varies widely and may be geographically related. Some authors have found few infected fish, if any, in the areas they have examined (Puerto Rico: Bunkley-Williams and Williams, 1994; southern California: Wilkie and Gordin, 1969) . Others have found that low levels of infection are not uncommon (e.g. southern Queensland; Diggles and Lester, 1996c). Keeping multiple fish in holding tanks and at aquarium stores increases the chances of a fish carrying "Ich" parasites, but it is still possible to acquire a fish that is not infected with "Ich".
If new fish are quarantined for at least 6 weeks, any parasites on the fish will have gone through a number of life cycles increasing the number of parasites present. In the majority of cases, the increase in parasite numbers will result in full blown infection and fish can be treated to remove the parasites. Hyposalinity has been demonstrated to break the life cycle of "Ich" (Cheung et al. 1979; Colorni, 1985) and fish correctly treated with hyposalinity will be free from "Ich". Any fish that do not show signs of infection after 6 weeks are very unlikely to be carrying any parasites.
If fish that are free from "Ich" (either because they were not originally infected or because they have been treated with hyposalinity) are added to an aquarium that is free from "Ich", the aquarium will stay free from "Ich" and be another exception to "Ich" always being present.
Burgess and Matthews (1994) were attempting to maintain a viable population of C. irritans which could be used in later studies. To maintain the parasite populations, they needed host fish in order for the trophonts to feed and continue the life cycle. Each host fish was only used once in a process of serial transition such that none of the hosts would die or develop an immunity. While the procedure worked very well and enabled them to maintain populations for some time, the viability of the populations decreased with time and none of the 7 isolates they used survived more than 34 cycles, around 10 to 11 months. They suggest this is due to senescence and aging in cell lines is well recognised in Ciliophora.
The presence of aging cell lines in C. irritans suggests that an aquarium that has been running for longer than 12 months without any additions is unlikely to have any surviving "Ich" parasites, yet another exception to "Ich" always being present.
Whilst "Ich" may be present in some aquaria, it is certainly not present in all aquaria. Through careful quarantining and treatment, it is very much possible to establish and maintain an "Ich" free aquarium.
That said this supports Dave's claim because as long as there is a fish living in the aquarium there is a potential for the parasite's life cycle to continue, even if it goes unnoticed by us. There is also evidence that fish develop immunity against the disease
Immunity
Innate Immunity
Innate immunity refers to the general response to an invading pathogen or parasite regardless of that pathogen or parasite encountered (Dickerson and Clark, 1996). This form of immunity does not rely on previous encounters and includes generalised reactions such as secretion of mucus, but may include specific host cell responses (acquired genetically).
While little formal study has been performed on innate immunity of marine fish to C. irritans, innate immunity of freshwater fish to I. multifiliis, both between and within host species suggests that the former may be possible. Collective anecdotal evidence from marine aquarists lends weight to the idea that some species, such as chaetodontids (butterflyfish) and acanthurids (surgeonfish and tangs) may be more prone to Marine "Ich" infections, whereas other species such as callionymids (dragonets) are not at all. Intraspecific differences in innate immunity would be much harder to detect through random observation.
Acquired Immunity
Acquired immunity occurs when the response is specific to the invading organism, which is recognised directly or through antigens (Dickerson and Clarke, 1996). Colorni (1987) first suggested that marine fish could acquire some immunity to C. irritans by surviving several infections. Burgess and Matthews (1995) demonstrated acquired immunity in the thick-lipped mullet, Chelon labrosus. They found that 82% of fish that had been previously exposed to high levels of theronts were immune to a secondary exposure.