In a .5 gal you should have nothing but the Betta, and even that might be too much... Otos need a minimum of four of their species to thrive and you need to have a bunch of algae and be able to figure out a way to get them to eat more when the algae runs out... And most plants would all outgrow a little tank like that almost immediately. But hey, I could be wrong. Someone else may have a more accurate opinion.
55 Gallon: Crowley: Melanoid Axolotl; Mischa: Melanoid Axolotl; Achlys: Albino Axolotl; Eddard Stark: Leucistic Axolotl
Furry Friends: Daisy and Holly: Dogs; Norman and August: Cats; Gilbert and Mordecai: Guinea Pigs