Betta Fish: i switched my bettas tanks

My first betta was my koi, and she was in the black tank. i later got the male, and he was originally in the white tank but i just switched them. i switched them because although they’re both 5 gallon tanks, the white one feels smaller because the filter takes up so much space (i regret buying this tank) and the blue betta is larger

I feel bad like i did my first koi dirty with a smaller tank, does anyone have ideas how i should decorate it better? and was there any problems with just switching them?