Stykthyn is right; there's reef compatible (there are a few corals it won't eat), reef safe (may be true at one phase of its life, then it grows up), and that's about the tale of it. The only fish you can trust with corals are those that never grow mouths big enough to nip a polyp, IMHO. Even algae eating fish may decide your, say, green tentacled candy cane coral is plantlike. The gobies and blennies and dragonettes are the most reefsafe. The lawnmower blenny does not in my experience graduate to eating corals, despite his huge mouth and reckless feeding: we had one for years that was perfectly well-mannered. Butterfly fish and angels survive by grazing along the reef and moving on, never doing enough damage that a coral can't repair itself---the only problem is, in a tank, they keep revisiting the same coral, again, and again, and again...and pretty soon it's irreparably damaged and the reef undertakers move in. Keeping a contained reef means we have to be really careful what we put into it...and what a dealer will tell you when trying to sell you a fish is generally a hopeful estimate of its behavior. Individual angels, for instance, can be perfectly well-mannered for months, until the day something clicks over in his fishy brain and he decides, while you're gone for the weekend and you haven't fed him, that your expensive goniopora looks enough like lunch.
