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.