I have to disagree. I keep a betta in my 55G community and a betta in my 29G snail tank as well as the betta in the 10G. They all use up every inch of tank space they are given. I do have a betta in a 5G and he doesn't swim around as much, but where is he going to swim to? If you give them the space and let them adjust, most bettas will like what they are given very much. You just have to make sure the current isn't too strong or they will have a hard time swimming. It is also a good idea to give them a place to hide, such as a cave.
I have a little red female Betta in a 5 gallon. She loves it, and has 3 ghost shrimp and ohhh...'bout a million snails (j/k) for company. lol She was in a 33 guppy tank for awhile. She loved the room but I think the guppy groupies annoyed her. (They followed her everywhere..it was cute). The current was a bit too strong too. She'd swim into the filter outtake and go flying across the tank. Poor thing. It was also too deep for her. She was used to that horrible little cup, not a deep tank. Got a bit frantic at first because she didn't allow herself enough time to surface before running low on air. :/
I myself keep a single betta in a 10 gallon. My betta seems very happy and swims around the entire aquarium all the time. I would hate to keep him in anything smaller.
One bonus to a 10 gal tank is it will not get very dirty. You fish will love ya. You can get an all-glass 10 gal very cheap these days. I have an UGF and small heater in it.
I have a betta that lives alone in half of a 20g tall. He is VERY active. He'll flare at any snails that sneak into his side of the divider. The other side is occupied by a german ram. I have a lot of filtration on this tank and plants, so it stays nice and clean. The two fish don't get along, so the divider is full of moss which every day is hiding more and more. Both like to pick at the moss and play in the plants.
Just a little note about seeing betas not "like" larger tanks. It can take a betta that has been jarred for his/her entire lifetime almost 3 weeks to fully adjust to more space and become fully active.
