Big fish, the main thing here is that you should not do the same post on more than one forum ok? Also you won't fit a monster in a 20g tank but there are a lot of small but agressive fish around you might like. If you want fish that eat smaller fish and stake out territories you might try some pygmy sunnys or some of the smaller african cichlids or even some of the little fw mottled sculpins that live all over western Washington, they are all quite amusing. The thing is just decide and get started, when it is up and running send us a pic or 2.