best use of 20 gal

I'm getting a 20 gal (for free, score) and the other day I saw an aquarium with a "shark" in it. The tank was much bigger than 20 gal but I was wondering if there are any I can keep in a 20 gal. The shark I saw looked like a mini great white, and until a mini great white actually exists I will have to settle for a freshwater looks-like-a-shark Shark to satisfy my craving to keep a small version of an animal I'm horrified of, or at lest that what seems like is goin on. Other suggestions are welcome, im also thinking just goin community but could also be talked into salt water. So basically I guess I'd like some opinions/pictures on what is the best 20 gal set up. I have another 20g with a Bushfish in it, his name is leonard (unrelated). I need some inspirado. I could also use some tips on diy sumps, and the right pumps to use and what not, Ive had bad luck (spills,overflows) in this area and I thought before I drive my girlfriend completely insane I should get some info, then drive her insane.

do not touch the trim!

Thank you Nick, you kick ass.

K im likin the sound of the bucktooth bad boys, how many can I get in that twenty gal. please nobody respond with a math formula, If i wanted to do math I would have succeeded much more in life by now.

Yeah I think with the quarterly ill be gettin that all set up. I just watched a video of those things and me likey. I think i just read something about they eat plants. Is that true because I like plants in a tank. And will they bite me if i put my hand in, because that would be great.