Bare minimum of 1 gal. per tank, but I haven't been able to find any that size with heaters, so I'm fine with bigger tanks. Probably around 2 - 2.5 gals. I have 2 male betta's and 1 female, and my bf has 1 male betta. They're all kept seperately so they don't fight and we don't end up with any little babies that we can't find a place for.