If you want a small predator, a marine betta is a good option. These guys are an awsome sight when hunting live food like glass shrimps. They swim at the shrimp, point their two front fins at it, bend their body into a U and start shaking their tail. As soon as the shrimp looks at the tail it's all over in one quick bite. If they are being defensive, they swim tail first at you with their fins spread. Their tail looks like an eels head.