i leave them on all the time - I'm too tight/lazy to buy another timer. You can only see them after the main lights switch off (did get a timer for them).
Nothing to do with fish, but they do throw a beautiful dappled water pattern out into the room, very dim and discrete, but it's lovely to sit there in the dark, with the room and the tank lights off apart from the moonlights, and watch the water patterns in the room, and the fish playing in the tank.... or maybe I'm just weird!
Sarah