moorish idols are tough, but they are not impossible. to be honest, all the responses besides for fishyvet's, have not contributed to the poster's original question. sure he didn't do the research, but telling him how these fish are impossible and how they are doomed in his tank is not going to give these fish a better chance at surviving?
wozz, how big are the idols? are they eating? here is a basic summary i've been to come up to successfully keep these fish.
these fish love lots of flow and it is one of the key factors to there survival. they need oxygen rich water and lots of swimming place. if you have alot of LR in your system, be prepared to remove most of it so the idols have a place to swim. As far as their diet goes, they will do very well if you can get them to eat a variety of food (clams, mysis, krill, flakes, nori, brine, pellets, sponge). be prepared to feed 3-4 times a day to keep them happy and if you can, supplement their food w/ vitamins and garlic. i'm sure there's more but this is all i could come up w/ on top of my head.
good luck. i can't find the long MI thread but i was very informative. if anyone has a link to the thread, please post!