Clown tangs are just about the worst choice for a reef fish on so many levels. Its very hard to keep one alive long term, they need an enormous tank (I'm not sure 240 is enough), and they can be among the most aggressive and potentially lethal of all reef fish. Their acanthus is huge and I've read it may be venomous as well and they can inflict grevious wounds with it. This fish does not get mellow with age, its gets even more aggressive and territorial.
I've always wanted one since they are truly among the most beautiful of all reef fishes but I will continue to resist that urge.