This is like asking if fishes sleep. Of course, fishes do sleep even though their eyes are wide open. The thing is that all living organisms need to rest for several hours in a day to help recharge their bodies. The question as to when you should turn off your aquarium lights is actually dependent on the type of fish that you have in your tank. Some aquatic species require light to retain their remarkable colors. There are also some species of fish that are known to become lethargic or will stop eating without light. So it is best to check the light requirements of the fishes in your tank.\r