All right, I'll toss a quarter in the slot.
I have seen ich once in the last 12 years. It went away, never repeated, through one house move and 4 more years. Hobbyists of my persuasion don't get the pest often and it usually doesn't amount to much if we do...a) reef. The water is kept spot on with an ato and kalk drip. b) no 'midwater' fish like tangs and angels, puffers, etc, all the thin-skinned types that seem to be ich magnets. c) mostly sand-dwelling fish like blennies, gobies, mandys, dartfish, or damsels. Damsels are, of that class, the most likely to have trouble, but not if they're not exposed.
My own theory: the ich life cycle involves the sandbed. Look at a reef cam, on a living reef. The midwater fishes never come near the sandbed. They're up and about, constantly hunting their food where THEY live. They have thin skins, super streamlined, not that great a slime coat compared to some. The sanddwellers, OTOH, living in the sandbed like jawfish, hunting close to the bottom like mandys (champion slime producers), and the little rock-suckers like blennies and hole-dwellers like gobies and dartfish ---are all inclined to have a nice slime coat. And to have natural defense against this pest.
OK. I've been at this over sixty years. I've seen tank-keeping mutate considerably, and we *used* to have a hard problem to dodge. What we have now is a LOT easier to keep disease free, because of the automations.
WHO in this hobby loses fish to ich? For starters---novices, who haven't got the automations yet, who don't know how to test their water (or who just don't), who don't know how to watch their corals for signals [in the sometime absence of tests], or who run fish-only tanks not because that's their really considered decision, but because they're new and they think those tanks are easier [from my viewpoint, they're far harder.] The combo of novice, inappropriate tanks, less apt equipment, inexperience at choosing healthy fish, buying cheap fish (from sources that aren't as careful themselves), and water quality bouncing all over the scale because of mistakes or lack of tests or topoff equipment---all of these set up novice tanks to be far from ich-free. One a person gets past that stage---sure, there are people who use a combination of skill and luck and don't have problems. Others seem to live under a ich-prone cloud...though I suspect its a gap in their procedures, one of the most glaring of which (purely IMHO) is the tendency to lose fish and replace them, each one of which is a chance for the pest to get in: I think I've acquired one fish in the last 3 years.
Quarantine rock and corals? Not this person. And again---ich just isn't a problem for me. It doesn't like my fishes. Doesn't find many chances in my tank. It's a poor dumb little lifeform that succeeds best where it has a multitude of chances; where chances like irritated gills and bad slime coat from low alkalinity are non-existent, it has a harder time, and if it can't find a host, that individual ich-let dies unfulfilled, without reproducing, within a relatively short time.
So, yes, we have quite a time with it among novices. And people learn to run scared of it. It's not super-bug. It is a problem if your tank has had a problem, and if you've just won the prize fish in the ich-prone lottery...at the same time.
