You can't get a grouper in your 15 gallon tank. But, if you want a bigger tank, groupers eat meat, frozen or live, they aren't shy. My strawberry grouper was so mean it was scary to his tankmates, but now hes at our lfs. :)
A nice tang, or angel would do nice, but come of those fish may get to big. So, you'd hafe to get a bigger fish, or get rid of your fish, when it grows up.
I saw some niger triggers that size in my lfs. They are usually nice, but some people on this bb got some bad apples, and yes they are extremely hardy.