I have 2 "moonlight" arrangements. One is a 4 lamp fixture that I placed in my 6 foot tank and the other is 3 LEDs that I placed in a 3 foot long tank. In both cases the light was just enough to give a nice blue glow and enough light for me to make out the fish outlines after the main lights went off. I left both plugged in at all times because the LED lights were invisible with normal tank lighting. It didn't seem to bother the fish at all. After all, where is there total darkness at night? There is always some moonlight or starlight in any natural environment unless you are in a deep cave or something.