Good carnivorous fish for a 20 gallon aquarium?

I have a 20 gallon tank and my last fish in it were 3 freshwater angelfish, ill never make that mistake again, they got a lot bigger than expected and didnt really do much, just sit around. so this time around i want to get a carnivorous fish, just to liven it up a little, i have a ten gallon tank to keep the feeder fish in. whats a good mean fish that will eat like crazy? i was thinking a puffer fish. I hate to sound all morbid here, but my last fish were just soooo boring.