Saltwater aquariums are lit using primarily a mixture of white and blue colored light, at least that is how it appears to our eyeballs. While some hobbyists prefer a whiter look and some prefer a bluer look, the reason we use this color of light is important. Not only is it critical for the photosynthetic animals (coral) in the tank but it also has an important role in providing the natural aesthetic in our aquariums.  It mimics the natural blue color that exists in the ocean. 

Spectral Graph

Light Spectrum

While the light appears to be mostly blue in our aquariums, there is in fact a full range of colors. Reef tank lights will advertise spectral graphs, as pictured above, which indicates the full range of color the light emits. Any reef tank light worth its salt will have a substantial spike in the blue and purple color spectrum. This is why the light looks blue but it is also an important part of the coral's physiology.  Corals have adapted to photosynthesize best under lighting conditions dominated in the cool color spectrum because this is what they get in the wild. 

As full-spectrum sunlight passes through the ocean's water column, warm-colored (red, orange, yellow) light is filtered out first. The deeper you go, the more warm color is filtered out. Corals growing on the reef and oceans floor are then left with a light spectrum that is dominated by blue and purple. 

It just so happens that blue, purple, and UV-colored light makes our corals fluoresce as well, which is simply an added bonus to using this cool-colored spectrum of light.  Various mixtures of blue, purple, and UV-colored light can be used to achieve that color-popping florescence we have come to appreciate in our aquariums. 

Choosing a Reef Tank Light

Remember this, your eyes are not always going to see the full picture. Just because the light looks blue, doesn't mean it contains the right spectrum for corals to thrive. This can only be determined with a spectrometer that measures the wavelengths of light.

Choosing a light specifically made for a reef aquarium is critical for the survival of your corals. These lights are manufactured to provide the perfect spectrum for corals while also giving you the ability to change the appearance from whiter to bluer. Even though some tanks might look white and some tanks look really blue, a reef tank light still needs to be supplying that full spectrum of light with spikes in the UV, purple, and blue wavelengths.