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.
I have a marine betta (grouper) in my reef tank. Hes been there about 6 mo now. He doesnt bother anything. I DO make sure I dont buy anything new he could fit into his mouth though. He eats silversides and chopped squid w/ zoe. I have heard they can begin to eat other tankmates, but I havent had any problems. (I try to keep him well fed and happy) He is even kinda shy, he hangs out under a ledge and cruises the back of the tank, unless its chow time. VERY COOL FISH.