Just a few observations gleaned with 7 years of keeping idols and seeing them on the reefs here in Hawaii:
Diet: IMO diet is not the problem with keeping idols ... they will eat and thrive on the same well balanced diet that most captive reef fish do.
Small ones: This is one of the keys ... getting them as young as possible so that they have not yet fully converted to a set diet on the reef is critical. At this point you have a better chance of getting it to eat introduced food. Not all will eat IME but most will given enough time.
Tank Size: the other key, these fish need a lot of room ... we have kept them in tanks from 500-7000 gallons for over 10 years, no problem.
Compatability with corals: IME they tend to leave SPS alone, but are fully capable of developing a taste for LPS corals, particularly
Lobophyllia.
Numbers: Unless you have a true pair, keep one per tank. Small ones together will eventually either pair up or split up on the reef. If they don't pair up in your tank, one will kill the other. I have never seen schools of this fish in Hawaii, either singles, pairs or sometimes trios.
Water Quality: I can't comment too much on this since most of ours are kept in open systems, but you can find them swimming in inshore waters as well as along the seawalls of Honolulu Harbour, so what do you think?
Aloha!
J. Charles Delbeek