In my opinion the ultimate "predatory" fish you can fit in there you and most people probably have never heard of.... I had one of these things and it was a straight evil little monster.... Full grown should reach 5" or so, does not get along great with others. I had some lemon tetras in the tank with this fish and they were fast enough to survive but guppies are not.... My favorite thing to feed it was ghost shrimp.... This fish is also extremely intelligent, you can watch it think about its next move. My hands down all time favorite fish..... The Ivanacara Adoketa. Good luck finding some though, they are expensive and rare.... But check out some videos of them. This is my own video here....
I had to leave home for a while (work) and sold off almost all my stuff, including this fish. I still talk to the guy that has it to this day. I don't think he plans on letting me have it back though.