Rather than give my opinion, I'll throw out this 2 part article.
Part 1
Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro - Reefkeeping.com
Part 2
Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part II by Steven Pro - Reefkeeping.com
"Fish can develop immunity to
Cryptocaryon irritans that can last for up to six months (Colorni, 1987 and Colorni & Burgess, 1997). It is this natural immunity that makes evaluating the effectiveness of various treatment options so difficult. How can someone ever be certain that what they dosed to their tank or fed to their fish is what caused the cure they observed? The answer is simple, they can't. Until there are controlled, scientific studies, preferably repeated a few times, we cannot be sure that any of the newer homeopathic or "reef-safe" treatments actually work.
This limited immunity is also the basis for some aquarists advocating that if a fish gets sick, to just maintain pristine water quality, feed a superb diet, and to allow the fish's own immune system to do the job. While it is possible that this could work, natural immunity is not totally foolproof. In the studies cited above, some of the fish were not completely protected by their own natural immunity. It is possible that immunity could protect the fish from massive infestation, but still allow small numbers of parasites to remain and reproduce undetected by the aquarist. This is where the 'Ich is always present' argument comes into play and why sometimes an aquarist has recurring difficulties with this pathogen. It is possible, in some cases, that the treatment was cut short or misapplied or for some other reason not totally effective, but that immunity helped to partially ward off the infection. In this situation, a low-level infestation, held in check by natural immunity but not totally eradicated, could continue to survive but be misdiagnosed, or missed all together, by the hobbyist. The parasites could progress through their lifecycle by predominately attacking the gill tissue, where they could go unseen. Or, the number of parasites could be so low and their appearance (and disappearance) be erratic enough that the aquarist does not pick up on the infection or attributes the occasional white spot to a speck of sand or air bubble because the fish are behaving normally otherwise. That is until some other minor mishap occurs or the immunity wears off and the balance shifts in favor of the infestation, resulting in a full-blown infestation once again.
I want to be clear on this point. I do
not believe
Cryptocaryon irritans is always present in our systems. With a strict quarantine protocol, it is possible keep an Ich-free aquarium. I just believe that there have been enough hobbyists who have misused a treatment or utilized an ineffective treatment option, such that they never really fully conquered their initial infestation. Their continuing problems over the course of many months, and the posting of those experiences, seem to be enough to promote this aquarium myth.
Cryptocaryon irritans can be eradicated from an infected system with a proven treatment and can be kept out of the system if good quarantine practices are employed.
Most of the hobbyists who consider letting their fish fight off the infection on their own are hopeful that it will work because they find the challenge of removing all the fish and allowing the aquarium to go fallow (without any fish hosts), very daunting. If you are considering natural immunity as a treatment option, ask yourself a few simple questions.
•How big is the tank?
•How difficult would it really be to tear the display apart to capture and remove all of the fishes to a separate hospital/quarantine tank for treatment?
•How much is it going to cost to replace all of the fishes in the event of a catastrophic loss?
•What kind of moral and ethical responsibilities do you have for the pets that you purchased?
•And lastly, what in the world were you doing adding anything that had not been quarantined into your aquarium in the first place?
I cannot answer these questions for you, but it is my opinion that it is inappropriate to do nothing. I would hope for acquired or innate natural immunity to kick in when used with other less aggressive but pro-active treatments, such as using biological cleaners, medicated foods, UV, ozone, and garlic. Although I clearly don't believe this shotgun approach of unproven treatments is the most effective option available. For me, if you gamble with un-quarantined items and infect your tank, it is best to bite the bullet, remove all the fishes to a separate quarantine aquarium, fallow the tank, and use a proven treatment."