For a marine fish only aquarium lighting is simple, the fish need to be able to see and be seen. For a reef system the lighting is more complicated, as not only do the fish need to see and be seen the corals will normally have zooxanthallae within their flesh and this single celled algae needs to flourish so that the coral does too.
There’s a form of lighting that presents not a single headache and that is moonlight.
In the wild as everywhere else there is day and night, plus the transitional periods dawn and dusk. At night there is often moonlight which is missing from most aquariums.
Having moonlight is not essential. Corals are not going to suffer from the lack of it. Having mentioned corals and moonlight, isn’t it fascinating the mass spawning of corals on a reef, all of them triggered more or less at once.
Moonlight is optional. For those well-heeled aquarists with LED (light emitting diode) arrays it is possible that it is built in and ‘on’ times can be programmed. For most of us it is an extra purchase.
When all the lighting is off except for moonlights the effect on the captive reef is lovely. Weak shafts of light descend onto the corals, flickering across them. It can make the night period quite beautiful and draw the aquarist to have a look at a time when usually he/she wouldn’t.
Moonlights are a small array of blue LED lights. Usually they are supplied in a small unit, with perhaps five or less LED’s. There are larger units. Sometimes they are powered by a plug-in transformer style electricity supply. Their demand for electricity is very low, so very little will be added to the electricity bill.
As said, putting moonlights onto an aquarium is optional as there isn’t a technical requirement. Very advanced aquarists could well experiment with them to see if corals could be induced to spawn though this would be difficult and time consuming. Most of us would use them purely for the lovely effect, and expand the period when the aquarium acts like a magnet.