You don't need to leave the light! Fish are pigs and they will find it on their own! Haha. You will notice if you watch the tank with the lights out that when they fish aren't resting, they'll be picking at the gravel looking for food. I do like to have the lights on when they eat though so I can watch them eat to make sure they are all eating, as non eating is a sign of stress and/or illness. Up to you though!