I have to say, the way that question is written doesn't allow for an accurate or correct answer to be given. The answer to that question is that it depends on the amount of work invested to provide the basic needs for the fish, and how happy you want it to be? Breeders keep bettas in mason jars, nowhere near 1/2 gallon, and they thrive... pet stores keep them in cups, and yes, some stores do the required work to clean them every day, and they do fine there also. The proper way to determine what is best for any individual fish is to take a look at its needs and what it will take for you to properly provide those. Bettas come in different sizes, too. The size of the fish also determines it's needs. A male fighter who is 3 inches long and an inch tall (yes I've seen them) isn't going to need the same as a crowntail that is 2/3 fins and 1/2 the size! What do bettas need? Clean water, warm temps, good and healthy variety of food, a place to hide or play, and attention. Can you provide that in less than 1/2 gallon? Sure... if you are selective about your fish and meticulous about your maintenance. Will your fish be happy? Play with it and find out!! Most bettas have issues more with boredom than tank size. Is it humane to put a betta in a 10 gallon tank full of decorations, heater, filter, etc. and igonre him? I've seen this done just as often as the small containers that are doted on, and I've seen prize fish come from every type of scenario you can imagine.