8 gallon halfmoon, 10 gallon with stand, or 5 gallon hex?

remember a betta is a labyrinth fish and requires minimal room. 8 gallon hex sounds nice but is a single betta REALLY worth 120$ to you? the used 10g is still a little large unless you are going to make it a community tank. the 5g is probably your best choice based on size, and most bettas are fine without a heater and minimal decorations, although they do prefer slightly warmer temps. hope this helps.