I had a betta live 18 months in a 1 gallon pickle jar (without the pickles) before he was moved into a 10 gallon. I think he preferred the jar, he eventually became lunch for the Monster.
This thread is almost 7 years old...i noticed this thread and thought i'd add my bit...
I recently bought 2 dwarf male gouramis for my 10 gallon aquarium. I have a 10 gallon quarantine tank, however upon filling it with water i noticed a leak. I can not use my hospital tank until my brother seals the leak and in the meantime i've had to use two of my empty betta tanks. These tanks are 1 gallon heated and filtered aquariums that i keep my bettas and the occasional feeder minnows in. They all have optional aeration that i put into use for the gouramis... i know they are labyrinth fish and all but bettas dont use gills AT ALL and other labyrinth fish use gills to varying degrees and i wasnt sure how much the gouramis prefer to use theirs. It's been a week and both gourami boys look surprisingly happy for such a small amount of space. They're almost done with quarantine and fairly soon they'll be in the community tank but they dont look at all unhappy in the 1 gallons.