Predatory fish or two for a 20 gallon tall?

[Asianleful]

I've posted this before but with a tank size error. The tank has a few rocks in there stacked up on each other to form a thing kind of like for African cichlids. There's sand in there and freshwater mussels. I'm looking for a fish that wont out grow the tank or will take a long time to do so. Thanks!