Many years ago, I had one outgrow my tank and I only fed it mostly Ocean Nutrition flake, which isn't available here anymore, with the occasional nori and Mysis and freeze dried plankton to supplement it. But I'm sure New Life Spectrum is all you need and some supplement.
Its been a while since I mentioned it on the forum, but here goes. The Idol is the most highly oxygen demanding fish I've ever kept. Angels were my 2nd most highly oxygen demanding fish I've kept. When I've unplugged my aerator/skimmer, after some time I would see the Idol laying on the sand gasping unable to swim, while the other fish are fine (including tangs and angels). It would slowly recover after I turned the skimmer back on. Would have loved to have recorded this on video for people. After my Idol was adopted, one time I left the skimmer unplugged overnight (with only powerheads and aquaclear providing circulation/aeration), the next morning, my well established angel was dead. All else was fine. I just couldn't get enough o2 into my tank (for high oxygen demanding fish) without an air diffuser such as the protein skimmer (I had powerheads and aquaclears going only). It was repeatable as it would happen once or twice more in future years every time I forgot to plug my skimmer back in overnight. It was always the angels, and nothing else, and always found dead the next morning after they were established for a long time. I had a DO meter and found that my morning DO levels were very low even with the skimmer on so without the skimmer running at night, it was just clearly lethal to the fish that require the most oxygen which happens to be the Idol (which luckily was adopted by someone before I had my mishaps) followed by my dwarf angels. Sorry, I know you don't want to discuss anything but diet here but I've been silent on this for a few years already.
I'm not planning to write about this topic too often so I thought I would try to get this in here. My feeling is that the diet requirement is a bit overrated and I know people will disagree with me. But I'm pretty confident I could keep another one and not be too concerned about the diet and it will outgrow my tank again. Given what I said, I agree that this makes the Idol hard to keep due to its high maintenance and low tolerance for mistakes and should be left to advanced aquarists.