Here's my two cents worth on bettas. The reason they can survive in a tiny jar or vase of rather dirty water is because in the wild, the waterways in which they live all but dry up for a part of the year. The bettas live in tiny pockets of, essentially, mud for extended periods, waiting for the rainy season. However, when the rainy season comes, they are in open water and they swim, mate, and find food. A 10 gallon tank is perfect for a betta, imo. I have some bettas in small tanks (8 & 10), but I also have a male in a 45 gallon community tank. It's well-planted with lots of wood and rocks, and the betta seems to thrive in there, despite some unorthodox tankmates. He alternatively sits calmly near the bottom or top, or swims in the open water, all his beautiful finnage on display. As someone else said, as long as he has some room to swim and an air pocket above the water level, a betta is okay in any sized tank.