The provision of light to a reef tank is a very important aspect and is only second to that of water quality.
However providing lighting is not just about purchasing a [tag-tec]metal halide[/tag-tec], fluorescent tubes etc and placing them above the aquarium.
We need to think about colour.
The light which we see is made up of various different colours which ranges from ultra violet, through the blues, green and yellow through to infra red.
In the saltwater aquarium hobby we assign what is called a colour temperature and this is rated in Kelvin.
The more towards the blue end of the colour spectrum we go the higher the Kelvin rating will be and on the opposite side the more towards the red end of the spectrum we go the lower the Kelvin rating will be.
So how is the Kelvin scale important to a [tag-ice]reef tank[/tag-ice]?
As said light is not just light but is a mixture of colours which when mixed with intensity (wattage) will supply the corals with the required energy they require.
Let’s take a look at the natural reef for a second. On the reef at the waters surface the Kelvin rating is around the 5000 – 6500 kelvin mark and the Kelvin rating increases the deeper we go into the water.
Think about where your corals are collected from. Are they from the surface, no they are from deeper down. Saltwater has the ability to rapidly strip out red from the colour spectrum when the light source passes through it and for this reason (and that of millions of years of evolution) the symbiotic algae in the corals have become accustomed to light which is at the blue end of the spectrum.
What we need to remember as aquarists is that we are not attempting to replicate the lighting as it is at the waters surface on the coral reef we are attempting to replicate the lighting as it is where the corals are located.
The saltwater aquarium is exceptionally shallow when compared to a reef!
For this reason the majority of aquarists utilise aquarium lighting which is between the 10,000K and 20,000K rating with 20,000K being even bluer than 10,000K. Personally I utilise lighting which is rated at 14,000K as I believe that this gives me a good balance of blue for the corals and being aesthetically pleasing for me.
In my opinion 10,000K is not quite blue enough and 20,000K is too blue.
Whichever Kelvin you choose to use just ensure that you provide enough blue for the symbiotic algae to provide energy to the coral so that it can grow.