depends on how high the divider is and how tight a fit the lid is, bettas are great jumpers when they choose to be, some never bother will others will have a go, personally i would have the divider at least 2" above the water level and even better if theres very little gap between the top of the divider and the tank lid if its got one, i dont know how high a betta can jump, but u can bet if they are gonna jump will do it the minute ur backs turned or overnight so u get a nice little surprise in the morning