Many thanks for the explanation. Also thanks for the QT note which is interesting as it seems to imply ich is not always aggressive but can be a slow spreading parasite? Which is interesting given its life cycle, speed of life cycle and crowded nature of a tank.
By which I mean one of the great mysteries I've always wondered is why ich is never seen at the lfs, it's always at home. Second that ich can only occur on certain fish types within a tank.
I must confess I have had otherwise healthy fish catch ich.
Last lot was introduced possibly when I got plants (in water) or cardinal tetras several weeks before that. The rasboras in the tank caught it. I would swear they were healthy. The catfish (one which is unhealthy), also a batch of platys (that are like canaries in a coal mine), loaches, mollies (even same ones that caught it two years prior) and barbs did not catch it or it was not visible. The cardinal tetras looked fine, then ich, recovered with no losses and look fine again. What I find interesting in that is that I can understand the fish slime coat beating it off but am unaware of any defences in the gills.
Based on my experiences and education, the fish you thought were healthy, ( The rasboras in your example) may have been 90% healthy but not healthy enough to fight off the parasite. Which brings up the question, "What was happening in the tank that those fish weren't 100% healthy?" The comment you made about "the crowded nature of the tank" may have a lot to do with why the tank and the fish weren't 100% healthy. Back in the day, crowding a fish tank was the biggest NO NO you could do with the exception of Mbuna African Cichlids. The cichlids were done this way to prevent harassment and dominance in the tank which in turn made the inhabitants less likely to be picked on thereby being less under stress. It's been the only example of successful overcrowding I have ever read about or experienced. You just don't see overcrowding in nature until the dry season has made the available water supply so low that the fish have no choice but to congregate in it and most of the time, these fish either die off or get eaten by predators. The lucky ones that do survive, suffer until they can rid themselves of the parasites.
There's another process that may come into play here as well. In an open system, a parasite can attach to a fish, get it's nutrition then leave the fish as it swims off into the sunset. After the appropriate amount of time goes by and the new theronts now need a new "victim", they attach to another fish, do their business then fall off. Both of these times, 1 or a few parasites have attacked a fish and then fallen off leaving that fish to gain back it's strength or meet it's maker. In every school of fish there is always a weaker member and most likely, they are the ones that get infected. In a closed system, the same fish gets attacked over and over which is more likely to bring an earlier demise to the fish because it's not left with any period to recover. This is why infestations can be massive and deadly in such a short period of time.
Okay, now for some secrets from an insider
The reason you don;t see ICH in a pet shop is that they would be crazy to keep sick fish in the public's eye. Have you ever seen a fish store's "Hospital Ward"? If you haven't, it just means the store is good at hiding it.
Sick fish come in from wholesalers, fish get sick at the store and some fish are carriers not yet infected. The store's job is to not let those fish go out to a customer. But we just discussed how the second stage of the ICH parasite is off the fish. So what's to say a tank of fish has come in and they get ICH. The store does the right thing and takes those fish out of the tank and moves them to the rear of the store to medicate them and they put new fish in the tank after changing the water. Does this mean no Tomont is left in the tank? Not unless the tank was sterilized. Just cleaned does not equal sterilized. So then a customer buys a "healthy" fish from that tank where they get some of the water. Bingo, the new fish is now infected and you may not see it OR it is not visible to you. End result, new breakout.
Certain fish are more likely to get ICH than others. That's because they are more stressed than others. Pictus cats are the worst I've seen. They get ICH at the drop of a hat. I mean, I love that fish but I never had them in my home aquariums for just that reason.
As I recently posted in another thread, in today's hobby, it's insane ( IMO, to be politically correct
) to not QT any new arrival prior to putting them into your display tank. No matter what fish it is. In a centralized system, as most of the big box stores have turned to, the only time you can consider a fish close to 100% healthy is if every other fish in that system is not sick or there has never been a sick fish in the system. Yeah right, like that can happen!
1 fish in that system can infect every fish in that system. So the need to QT is paramount to preventing diseases in your main display tanks. Same goes for plants. As you've read, the Tormonts attach to the plants, so they can be carriers. Snails are also well documented carriers of parasites. So NOTHING is really a safe bet to not QT.
I hope this helps
