The whole 'Ich magnet' thing raises so many questions. The 'experts' keep telling us that we must quarantine all fish to ensure they are healthy before putting them into the tank, yet time and time again, we read that after quarantining fish still develop Ich ! So, using a QT is not 100%.
There is a difference between QT'ing and medicating tanks. Yes, time in QT to observe the fish is a good thing and recommended to do. But just QT'ing a fish isn't going to kill any parasites. Medicating a fish in QT will. People use the two interchangeable and thus there is confusion about it. And yes there is a lot of conflicting information on the subject. This is partly because many people have many different opinions on the subject and different experience. Many information shared is anecdotal. As with much information in the age of the internet, you need to put on your critical thinking cap on when deciphering peoples experience and opinions.
If you have a fish that is known to be more susceptible to marine ich and other protozoa or disease it is advisable to treat with meds in a QT before placing in your main tank. Not just observing them. Some people medicate all their fish for a time before adding to the main tank, and other just observe them in QT. Some facilities will treat their fish (see Divers Den medicating procedures before fish are available for purchase) and others won't.
My opinion, and this is just my opinion, is that Ich can be looked at as something similar to cancer in humans. As humans, we all carry cancer cells. For most, this never affects them, but for other something triggers the cancer, and it starts to grow and eventually kill the human. Let's not forget that Cancer cells are also living things. I think Ich in fish has something in common with this, and that for most fish Ich is always there, but just lays dormant and is never an issue. For other fish, something triggers the Ich to start growing.
Um, no. But I see where you are going with this analogy. It is an interesting way to try to make sense of what you are seeing that is happening. Unlike cells in our own bodies that are always there and can change, marine ich is a protozoa and isn't always there. Protozoa are single celled organisms, not part of our own bodies. Protozoans separate living organisms. Many call them parasites because they are an organism that lives in or on another organism (its host, in this case a fish) and benefits by deriving nutrients at the host's expense. This is why it is different from cancer.
But, what you are also hitting on (I believe) is an organism living with a parasite (or cancer in your analogy) is always there. But, it is always there only if the fish is not treated with medication to eradicate the protozoa. Just watching a fish in QT with marine ich that you can't see won't eradicate it. The marine ich protozoa is in such low numbers that you aren't seeing it and the fish is living with it b/c at low numbers it doesn't necessarily cause ill affects to the fish.
What some discussions have been hitting on is the ability of the host (fish) to live a certain amount of time with the protozoa, which may be the case in the wild where the protozoa isn't able to build up in numbers that overwhelm the fish (some are using the word immune). But, in closed systems such as our tanks, it only takes time for the protozoa to build up in numbers where it literally consumes the fish host (affects the outer body of the fish and more importantly the gills) and spreads to other fish hosts in the closed system.
In addition, stress is known to weaken an fish's immune system which also allows for the protozoa to easily take over and spread.
Some 'experts' say that Ich can only be introduced into a tank.
This is true. If marine ich is not in a tank b/c all the fish have been medically treated to get rid of it, then an introduction can lead to a new break out. If it was never in a tank then you shouldn't see any breakouts. The issues here is that one little protozoa living on a host can do so for a while and a full blown breakout isn't necessarily seen for a while.
Is it a case that in reality it is always there, somewhere?
No, marine ich can't live without a host. Understanding the life cycle of the protozoa goes a long way towards eradicating from a tank and making sure all new fish go through medicated treatment before being added to a tank.
I know we've spoken about this many times in various threads, but for me this is the one area that really needs to improve in our hobby, the understanding, identification and elimination/control of disease in fish.
Fully agree. Many articles and posts mention that QT'ing is needed. And many take that as the answer all their issues or a preventative means for eliminating disease and parasites. In reality, true elimination happens with the fish is treated with meds to make sure that you are killing an unseen parasite and/or possible disease. As I mentioned before there is a difference between QT'ing and medicating.
What someone like you and me can do is read up on the fish we are getting. If the fish known to be susceptible to marine ich (aka an ich magnet) or is the fish known to have internal worms in its gut (aka many wrasses), etc., with that knowledge we can then set up a medicated tank for the fish and treat accordingly. This can go a long way towards keeping healthier fish and our main display tanks.
What happens fairly often is that when someone sets up a main tank a fish gets added and once that first fish is introduced to the main display tank without being medicated then any introduction of parasite or disease can occur. Any following fish introduced who is more susceptible can obtain the protozoa. And it is just a ticking time bomb where in time some issue might arise.
Now back to the immunity discussion, if you would like to join an older conversation on this topic, and I think you would, since you put much thought into this post, check out these threads:
Treating marine Ich in main tank
A discussion on Immunity
Happy to have more discussion on the matter.