Ok, looking for help. I just picked up a 20 gallon tank at a tag sale. I went to a couple other sites, looking for some predatory, aggresive fish. I was given some ideas, only to do reasearch and realize they wouldnt be good candidates, either they were too big or I was at the minimum tank size, which I dont want to do. So my question is what is a good predator, aggresive fish I could have in a 20 gallon? Also I want to use sand as a substrate. DOes this mean I cant use an undergravel filter? Thanks for any help. Ive come to trust this site a lot as Ive slowly gotten back into fishkeeping for the first time since I was a teenager.