Lighting is mostly for people, not the fish. I usually keep my lights on during the afternoon for a few hours. You can leave them on as long as you want during the day, but you'll just end up burning your bulbs out quicker. It's best not to turn the lights on after it's dark out also, more so if the room the fish are in is dark. You'll see them run to the hills to find over due to the fact that they have no eyelids.