What kind of fish should I get for my 20 gallon long tank?

It will be just the one so I can keep the tank just the way it likes if possible and I'd prefer something bigger than 4" if possible (don't exclude those options I'd still like to know). I have a classic eheim 2213 canister filter, fake plants, heater, gravel, light(2 24Ws), top cover (mostly like 2cm open on the back and front), and 2 air stones. Just makin sure you guys know what i have to play with.