Freshwaters for a 20 gallon long?

[Raven01]

Okay, lol. I just know some people who got a terrarium and had a baaaad time when they left their house...

You can't really get anything big in a 20 gal. A school of tetras would look awesome though. You can mix the different species of them and they'll all mix.

20 Gallon is way to small for any larger fish.
A school of tetra's is a good suggestion(Congo or Emperors look very nice), they can be nippy though.
Rainbow fish or small to medium gourami pairs is another possibility.
A redtail shark is an unbelievably hardy minnow but, territorial so would be rough on tankmates in a 20 Gal tank.
Butterfly/Ram cichlids or Kribensis work in a 20 Gal as well.

Edit: brackish tanks allow things like Archerfish and they are just cool to watch. Messy but, cool.