I leave mine on 24 hours a day. I have a 6ft tank and I only light a one foot section on each end of the tank. I figure this way the tank is not totally dark, but there are dark areas for the fish, especially in their live rock caves. I haven't seen any side effects leaving them on all the time. I have a second set that I wired to light the whole tank. I plug that in if I want to do some night viewing.