None.
Neon tetras are schooling fish and should be kept in groups of at least 6. (Any less isn't a school.)
6 tetras are way too much for a 2 gallon.
EDIT: A betta. =]
You would need a low power filter (sponge filters work best), and a heater (ideal temperature is about 78-82 F).
Perform 20% water changes every week or so to ensure that water quality is good. Feed about 3-4 pellets a day, 6 days a week.
