1) The Tea(bag) Party and and/or faith-based federal policy agendas;
2) The guys in the van parked out front of your house;
3) Caleb's daughter when she's 16;
4) Religious types on your front porch;
5) Proselytes in proximity to young children;
6) Anything 'On Sale';
7) Global warming as the 6th major extinction event in the last 750,00,000 years;
8) Used mattresses;
9) Fast 'food':
10 My complete lack of humor and inability to comprehend sarcasm.
This list is by no means definitive or complete.
The only thing I've personally encountered in almost 3 years with 30 tanks now, besides a single outbreak of ich, is a bout with fungus and cloudy/pop-eye, the latter two both in the same tank. I would highly recommend keeping bottles of Melafix and Pimafix on hand if you don't already. As far as the ich goes, a fish that's down will be more vulnerable to complications than a healthy one, but ich does its work pretty fast so crank the temperature up to 89 degrees over a couple days and/or hit it with malachite green. If my understanding is correct, there's ich in your tank now even if it's presently only showing on the one fish. Andrew's question implies that stress may have triggered the outbreak in the one with symptoms, which I didn't know/haven't heard of before, but in any case the wee beasties are in there now just waiting for another opportunity.
You probably know all of this already but would hate to see you isolate and treat just the one fish only to discover anon that the population of the whole tank is infested. Can't say why it didn't manifest during quarantine - something perhaps to do with dormancy in the life cycle, or perhaps it vectored in with something else like plants? Or am I totally off-base and ick is as ubiquitous as the cold virus and just awaiting opportunity? Take it away Andrew...
Not a big deal so much as a pain. But luck all the same.