All aquariums have a daylight system. These must be turned off at night for your fish to experience normal day/night cycles. The problem is you can't see your fish! The R2 Solutions Moonlight replicates moonlight in a tank and creates a great ambiance at night in your aquarium for viewing your fish (a beautiful, blue, shimmering, moonlight ambiance in the room) and is healthy for your fish as it mimics the light of the moon which influences feeding and spawning behavior. The compact size allows for the R2 Solutions Moonlights to easily sit beside conventional daylight lighting systems.