at first like the first week they dont swim well,, they kinda dart up to the top then sink back down, its normal,,and yes turn the light off at night, if they have natural light try that instead of a lamp espicaly a really bright one, cause i heard of somthing about damaging their eyes if its on for a long time,, like all day and night,, but i cant be sure of that info.
do you have any plastic plants that would float on the surface? my babies like to lay on the plants, pluss its not far for them to swim for air.