In my case it is essential. My mandarins only lay eggs under the moonlight. If it is dark they are hidden in the rock and nearly all white and if the main light is on they don't spawn. Under the moonlight however they do their gracefull spawning and it is mesmerizing to watch.

Without the moonlight, part of their life would be missing. I suspect that it is also a part of coral life as often coral will spawn under the moonlight. I think something in the natural life of the aquarium would be missing without it.

Mine only stay on until midnight but for that hour my mandarin are spawning on regular basis.