That's very helpful material, Mike and Kurt. Thank you.
In principle, I see the reasons for QT, and I agree that it sounds like a good -- probably indispensable -- precaution against a problem like the one that I'm having now. In fact, I will continue to QT new fish when I have re-started my tank, and I'm sure that I will be able to do a more effective job of it in the future, based on this discussion.
The trouble that I have with QT-ing is this: My recent experience with ich suggests to me that no amount of time in QT is enough to ensure positive results. Or at any rate, that the period of time required for a truly effective QT is so long (3 months or more seems to be indicated by some studies), that it really isn't practical for me, in terms of: 1.) space for QT tanks; 2.) cost of upkeep for multiple systems; or 3.) enjoyment of the hobby.
So, I'm still pretty discouraged with the whole thing. It feels like a never-ending crapshoot, which might be a good idea, but which also might be a waste of time, and you can never tell which until after it's too late to do anything about it.
Regardless, I will soldier on, as Jimbo suggests. I mean, I will soldier on after I restart the ****ed tank this weekend, because I'm not willing to sit around and stare at an empty tank for 3 months.
What is prettier than a lemonpeel angelfish? So yellow...