Maintaining a coral reef tank is considerably more challenging than keeping a few tropical fish: the water chemistry, temperature, light and many other factors have to be optimal. Not noticing that the skimmer has stopped working or that the nitrate levels have risen are the kiss of death to all coral species.
However: if you're prepared to put in the time and thought (and never be away from home for longer than a week!) the rewards are terrific! We've had our little 'cube' for over a year now, and learned what we can and cannot keep in the system: generally speaking, polyps and discs do well and - touch wood - so do Duncans. (These are the green and pinkish fleshy hard corals on the left of the tank.) Although you don't really want algae in a reef tank, some of the green caulerpa species are highly sought-after and attractive, so it's great that several types have spontaneously appeared.