What I don't understand is how the parasite can live in an aquarium without attaching to a host. I've been told that ich can't continue it's life cycle without a host.
It hides in the aquarium via the host actually. It cannot remain without fish to live on but does not always manifest itself as an outbreak of spot in plain sight.
There are several factors that work in a healthy fish's favor with ich.
First and foremost the slime coat which protects the fish from the freeswimming ich, and secondly a fish's immune response. Immune response is something I'm just starting to learn about, but basically fish that live with ich in the tank and have suffered an attack (so to speak) tend to be more resistive to big outbreaks later. This is one of the things I'm trying to research for part two of the ich article, but haven't gotten far.
Now for the hiding ich issue. Since the skin is fairly well protected and will ward off most freeswimmers, then the only real vulnerable spot on a fish is the gills. Gills are a preferred spot for most external parasites, they are thin, tender, have no slime coat, and carry a lot of oxygen rich blood very close to the surface. The problem is we can't see what is on a fish's gills unless it hangs out or causes so much damage as to draw our focus. When an ich tomont hatches it litterally releases hundreds ( con be more than 2000) freswimming theronts. so each ich spot you see on your fish potentially becomes 2000 + or - freeswimmwers. Only one of them has to find the gills of your fish and successfully attach itself to continue the life cyle. So even with fish that are highly resistive to ich, if one tomont hatches, the chance of none of those freeswimmers finding a gill is really slim. Since the freeswiming stage is the only stage where the ich parasites are vulnerable, and it is a short window amidst several days of life cycle, treatment needs to be kept at full gusto for an extended period to ensure that all ich is killed. if one survives, and then another and so on the life cycle continues indefinately, and largely unnoticed. One ich cyst on a fish gills will not cause much damage, but 3 or four every two weeks for an extended period will eventuallycause enough scarring and damage to have an effect. Not to mention that if some other source of stress (i.e. drop in temperature) weakens your fish's immune system you suddenly have an outbreak on your hands. This is where the Myth was spawned and is perpetuated. The marketing of meds doesn't help with this because customers want results to be instantaneous (me included) and the marketing folks know how fast ich will falll off of the fish, so they can promise results in 4-5 days, and most the time the appearance of success is there. I fell prey to this many times for many years, I also lost a lot of really beautiful catfish over the years. I have not kept loaches, but it is my understanding that they are as succeptable to ich as my pictus cats are, and I generally consider them the same when discussing treatment. But I know for a fact because I have seen it in my tank that even the most succeptable fish will develope enough resistance to make ich seem gone, only to have it rear it's ugly head later.
I hope that helps and clarifies, and assure you that my opinions and preferred methods are not the only ones that work, but there are many reccomended methods that just can't work because of the way ich lives.
Watch for flashing with your fish. if they flash even once a week, there is cause for concern. Ich is not the only cause, but a healthy fish will not flash or scratch, every time I convince myself that some fish just flash or scratch for fun, I later find out there was a problem.
Dave
