Good Animals for a 20L:
Turtles:
spotted turtle (one)
mud turtles (eastern, or striped... you can possibly keep a pair)
cagles map turtle (possibly a small pair)
Snakes:
Ball Python (one small male)
Hognose snake (one)
Kingsnake (one)
Milksnake (one)
CayCalkers Boa (one small male)
ummmm.... actually there are alot of little snakes you can keep... so I'll stop at that.
Lizards:
Crocodile skinks (trio)
Leopard Geckos (trio)
Rankins Dragons (pair, and thats cause I still think it too small)
Misc Lizards; (green anoles, grass lizards, tokay, etc...)
Crested Geckos (trio, but I would of perferred a 20 tall)
Others:
paddle tailed newts
spotted newts
tiger slammander
mudpuppys
surinam toad
sirens
alot of stuff... I would offered you some CB tentacles if you had a 20 tall instead... but oh well.