most people who say their bettas are unhappy in big tanks, don't put a lot of decor. my niece's CT came from walmart. in a nasty cup. lived in the 10 gallon for months. i kept it planted up with lots of fake plants, a hidey cave, and baffled the filter with another plant. happiest boy in the WORLD! his fins grew longer, he darkened up real nice, and was building THREE bubble nests at the same time. 8I
a bored betta, is not the same as a happy betta. they're SUPPOSED to swim. and move. and spread their fins. but, pet stores here in the US just care about money. and that means selling lots of little tanks and false info, rather than good tanks and correct info. because people will come back for more fish, and medications, when their fish in the tiny tank gets sick. but won't because their fish in the big tank isn't sick.