Lighting For The Reef Aquarium

Proper lighting is one of the most important aspects of any reef aquarium. The zooxanthellae that lives in coral tissues and gives them much of their color, depends heavily on natural sunlight to flourish in a symbiotic relationship with the corals. The lighting we provide for the aquarium needs to simulate this as closely as possible. There are many options when choosing a light source for the reef aquarium which can be confusing. What are the benefits of one system such as HO, VHO, T5, T5HO, and HQI over another? How much lighting is required for for proper coral health? What is do all these terms mean, 10,000K, 420nm, lux, and lumens? These are terms used when touting lighting fixture specifications, which we will explain in more detail.

Lux is a derived unit based on lumen, and lumen is a derived unit based on candela(foot candle). One lux is equal to one lumen(a measure of the intensity of light) per square meter.

To simulate the lighting for the reef aquarium we need to understand the light that occurs naturally on the reef with sunlight. The ocean has a blue color because it is the blue part of the spectrum that penetrates the water the deepest and the red and yellow are filtered out. Corals found in shallow reefs and tide pools receive more of the full spectrum and intensity of natural sunlight, while corals found in deeper parts of the reef receive more of the blue spectrum and less intense light. The wavelength of light is measured in nanometers, with violet and blue at the lower end of the visible spectrum 320nm and the red is at the higher end at 700nm.

Coral health requires photosynthesis in the zooxanthellae which intakes carbon dioxide and produces oxygen and carbohydrates for the corals. The chart shows the spectrum where photosynthesis is the greatest. Lamps with a higher red spectrum tend to grow undesirable algae while the blue spectrum benefits corals and coralline algae more.

Fluorescent Lights

Today's lighting technology for reef aquariums take advantage of the spectrum and intensity required to keep healthy corals and stimulate growth. The new power compact and T5 fluorescent bulbs have become more efficient than the standard and VHO(Very High Output) fluorescent bulbs and tend to produce less heat than the metal halide bulbs. The best benefit of the T5 lamps is that they are much cheaper to replace than power compact and metal halide bulbs.

Actinic Lights

Actinic lights simulate the penetration of the blue part of the spectrum and promotes the growth of Zooxanthellae algae. These lamps are mixed with daylight bulbs or metal halides in the 6,000 to 10,000 Kelvin degrees. The color temperature of light is the ratio of red to blue light waves measured in degrees Kelvin (K). At 6000 degrees (K), the ratio between red and blue is equal. The higher the content of blue light waves, the higher the color temperature. During the day, the color temperature of sunlight varies. It also changes at different depths underwater as reds are filtered out more rapidly than blues. Actinic bulbs are rated in nanometers which is the wave length which they peak. The most common are 420nm and 460nm and are available in the standard, T5, PC and VHO.

Metal Halides

Metal halides known as HID and HQI produce high intensity lighting that penetrates the water deeper than fluorescent bulbs. The Metal Halide systems are great for corals that require intense lighting. SPS corals, clams, and carpet anemones would best survive in a system with metal halide lighting. Many hobbyists will supplement metal halide lights with VHO or power compact bulbs to simulate a dusk/dawn effect and to add more blue coloration to the tank. The new HQI bulbs with a color temperature of 10,000K, 14,000K and 20,000K lamps afford a reef keeper the opportunity to provide captive corals with higher color temperature light, thus more closely simulating conditions on a natural reef.

Lunar/Moon Lights

A recent trend in marine aquaria has been the addition of lunar lights, aka moon lights. These are low-intensity blue or white lights that are intended to simulate the moon's light.
Breeding cycles of fish and invertebrates are closely linked to lunar cycles, so it is believed that the use of lunar lights helps encourage breeding in captivity for many species.
Also, fish are not typically subjected to complete darkness in the wild; there is usually a small amount of lighting from the moon at night. When they are subjected to complete darkness in captivity, this can cause some stress, which can have a negative effect on the health of your livestock. Adding lunar lights helps to combat this. They also are great for viewing nocturnal activity that you would not otherwise get to see. Since they are typically LED bulbs, they have a virtually infinite life span. They come in two colors, blue and white.

Choosing the right lighting

Choosing the right light fixture for your reef aquarium depends a lot on the type of corals you intend to keep. Generally, the VHO (Very High Output) fluorescent , PC (Power Compact) fluorescent and T5 fluorescent lighting systems are great for corals that require low to moderate lighting, such as leather corals, mushrooms, polyps, and LPS corals. The Metal Halide systems are better for corals that require intense lighting. SPS corals, clams, and carpet anemones would best survive in a system with metal halide lighting. Many hobbyists will supplement metal halide lights with VHO or power compact bulbs to simulate a dusk/dawn effect and to add more blue coloration to the tank. Moonlights can also be added for nighttime viewing and beautiful nighttime affects. As a general rule you should have between 2 to 5 watts of lighting per gallon of water. Again this depends on the type of corals you intend to keep. You should also count on a 12 hour cycle for best results.

PAR - Polysynthetic Available Radiation.

While PAR might be an unfamiliar term in comparison to the more often seen Lumens or Lux, it is by far the most appropriate way for us to assess light in connection with the reef tank since it measures the light intensity of the spectrum that is utilized by the corals. Lumens and Lux are measurements of how bright a light source appears to the human eye and as such has little relevance in the reef aquarium. I believe that a PAR meter is an instrument that all serious reef aquarists should own. Buying one that has a submersible sensor allows you to measure the amount light that’s actually reaching your corals. This in turn gives you the information you need when positioning or repositioning your corals to ensure that all the various species are receiving light that’s optimum for their requirements.