So, I am one data point, but I do have a long-lived Idol. I am a lazy aquarist. My water parameters are sometimes OK other times poor when I don't bother to change the water for a long while. Then I might do a bunch of changes to bring it back. Sometimes I run carbon, sometimes not. Sometimes I dump my skimmer, sometimes not. I keep my salinity in a range, but not at a specific number, and it may change around over time. I don't generally check my water parameters, but I am sure my nitrates are through the roof. Feeding-wise, I mostly feed pellets. Sometimes I get on a kick and feed lots of different types of frozen foods (usually when I get a new fish that I am trying to get eating), then other times I feed nothing but pellets for long periods of time. I have never fed sponge. I medicate with copper, with Prazi, and with Paraguard from time to time when needed. I have never quarantined. My Idol has been through all of this. Over nearly 8 years. In fact, my Idol has survived Ick outbreaks that wiped out other supposedly more hardy members of my tank. It is now the oldest fish I have.
So, the question is why has it lasted so long versus others that haven't? I am not sure. Maybe it is an exceptional individual. Maybe I notice signs that others miss, since I have been keeping fish for 43 years, so maybe I react to the changes that others miss. Here are the things I think I do know:
1. I do know that mine arrived healthy and in great shape, and I do believe that shipping is a major issue with these fish.
2. I do know that they need greens in their diet. Whether algae on the rocks, byropsis, or Nori, they need it. Sponge? Not that I have seen. But if they don't have greens for a while, the pennant stops growing, or at least slows down. A healthy Idol's pennant grows like crazy, maybe a half inch a day. It will stop, but if you feed Nori, it will start growing again. I have seen this over and over.
3. I do believe that they need a lot of room. I have an 8ft tank, and I think that helps.
4. When young, they need to eat all day long. They don't eat much at a time, so they need to graze all the time. If food isn't available all day, that might do them in.
5. They are somewhat timid, and need to be in a tank that has no aggressors.
So, that is my 2 cents.