Idols are extremely sensitive fish and extremely picky eaters. More often than not, they just do not adjust well in captivity and simply waste away in our tanks. Most people are not equipped, knowledgeable enough, or have the time to dedicate to give this fish everything it needs to be a happy and healthy tankmate. In the ocean, they would eat mainly sponges, tunicates, and small invertebrates.Should you find a specimen that is eating, be aware that they are voracious eaters and often they will become destructive.
This is a fish that most agree is better left in the ocean. Ordering or buying these fish from LFS or online just encourages these places to continue catching and stocking them, most often leading to a slow death in inexperienced aquarists' tanks.
I would suggest a Bannerfish of some sort as a substitute as they look rather similar. Heniochus Acuminatus is a beautiful and graceful fish that shares that high banner-like dorsal fin. This species is not reef safe however. H. Diphreutes looks extremely similar and IS reef safe. A third species H. Singularius I can't really find whether it is reef safe or not, but I have one and it hasn't touched my corals. There are also more species of the Heniochus genus that do not look as close, but are beautiful none the less.
No matter which you go for, the Idol or one of the Bannerfishes, just be aware that these guys LOVE to eat and need fed multiple times a day. Also tons of live rock to satisfy their grazing habits. (My Banner likes algae sheets as well).
Good luck!