All marine plants are photosynthetic organisms and require light to survive. Without the proper lighting photosynthesis will be impaired and the algae and plants will slowly die. The conditions that marine algae are subjected to in captivity are very different from those found in their natural environment. A typical aquarium is both nutrient and light deficient so marine plants must adapt to live under these artificial conditions. Without question, the use of some form of natural sunlight is the best way to grow marine algae and plants in an aquarium, but for most aquarists this is not an easy arrangement. Thankfully there are many capable artificial lighting systems that can achieve a similar spectrum necessary for photosynthesis. Most aquariums today feature a single full spectrum fluorescent bulb, but although it will illuminate the tank inhabitants, it is grossly insufficient to grow much of anything. Providing the proper lighting is an area that many hobbyists fail with respect to growing marine plants. Instead of providing a lesson on how plants and algae convert light to energy, we will focus on the proper spectrum and intensity needed for successfully growing marine plants in the aquarium. 

Spectrum

Spectrum ratings are listed by the Kelvin temperature scale.  The lower end of the Kelvin temperature range bulbs are most suitable for growing marine plants and are available in 5000K - 6500K. If your aquarium will include soft corals, most marine plants will also grow under 10,000K lighting. This is normally a more pleasing spectrum to the eye, as the lower temperature bulbs can appear green or yellow. Unlike terrestrial plants, macro algae react differently to varied spectrums of light and the changes in color among different species can be quite dramatic. There are a few varieties of red macro algae, for instance, that can be found in shades of red, yellow and brown within the same species. The distinct coloration is caused by the pigments associated which each species and the spectrum of available light it receives. Overall most macro algae and sea grass will do well under full spectrum lighting or 6500K. The exception is with some varieties of red macro algae, which prefer a much bluer spectrum and can benefit from actinic lighting in the 8000K range. Mixing bulb color temperatures is preferred and is the most appealing while still allowing for optimum growth.

Intensity

The same rules that apply to freshwater planted systems and some soft coral dominated aquariums translate to the marine planted aquarium and/or refugium. As a general rule, most marine plants and algae need roughly 3-4 watts per gallon of full spectrum lighting to grow. Of course, this formula has certain limitations and must be adjusted to allow for very small aquariums or those that are deep. Also, if using a high output fixture (vho, metal halide), less watts per gallon are generally needed. A few deep water species can do well in low light conditions, but will grow much slower in this environment. Sea grass however, need large amounts of intense full spectrum light, as much as 10 watts per gallon to thrive.

Duration

Most tropical regions receive approximately 12 to 14 hours of daylight, with 8-10 hours of intense light, followed by 10 hours of darkness each day. While certain macro algae can receive constant illumination, such as Caulerpa, the benefits are not well documented and growth rates are often affected. Most macro algae and all terrestrial plants need a time to rest as they still continue to respire after photosynthesis has ended. Generally a duration of 12-14 hours is sufficient for growing macro algae and sea grasses. Unlike terrestrial plants, algae need a steady duration of light in order to undergo photosynthesis. A simple timer is all that is needed to maintain a constant, stable environment. If growing marine plants in a refugium, it is generally acceptable to have the lighting on a reverse cycle so that when the main tanks lights are off the refugium lights turn on. This is reported to help with the stabilization of ph levels and the overall safety of its inhabitants as marine plants absorb oxygen during the night, effecting the ph.

Suitable Lighting

The most common types of fixtures available to light a refugium or marine planted aquarium include normal output fluorescent, high output fluorescent, power compact fluorescent, T5 high output fluorescent and metal halide light fixtures. The brightest are the metal halide fixtures and the least bright are the standard output florescent fixtures. While these are all suitable for growing most varieties of macro algae some will produce more growth and improved coloration with more intense fixtures. LED light fixtures are a new technology that a few reef aquarium hobbyists have begun using with good success. While this type of fixture is still in its infancy, it may in time become an economical option for providing photosynthetic organisms the light they need with drastically reduced energy consumption and heat exchange. In addition to selecting a proper fixture, the most efficient bulbs have a high CRI (color rendition index) or PAR (photosynthetic active radiation) value and offer the hobbyist the most beneficial light available to marine plants. 

Compact Fluorescent

This is the most economical lighting option for growing marine macro algae and sea grasses in the aquarium. They have roughly 3x the output of standard fluorescent bulbs and are efficient at providing intense light in small spaces which make them ideal candidates for use over a refugium. The bulbs and fixtures are relatively inexpensive and available in a wide range of wattages and color temperatures. 

High Output Fluorescent

These include very high output (VHO) and high output (T-5 HO). VHO lighting is an excellent fixture for growing marine plants, but has recently been eclipsed by the popularity of a newer technology, the T-5 high output fixture. Similar in intensity to the power compacts, the advantage of the T-5 is that the amount of light is spread more evenly over the aquarium. They are also more economical and take up far less space than traditional T-12 and T-8 bulbs. The bulbs are available in a wide range of spectrums including those in the 5000K - 7000K range.

Metal Halide

For a large marine planted aquarium there probably is no better lighting source than the use of a metal halide fixture. Metal halide offers the aquarist the closest spectrum to natural sunlight available and adds a shimmering, rippling effect that is not duplicated by any other type of bulb. Depending on the size of the tank, metal halides are available in single ended and double ended bulb styles. Double ended or HQI fixtures are the most efficient and generate considerably less heat than the older probe start ballasts.

LED 

High powered LED lighting is a fairly new technology for the aquarium industry. They are very efficient fixtures, using about 70% less energy than a typical metal halide fixture. Because they are not actually bulbs, they require a lot less power to run as well as much less heat. The fixtures can be mounted closer to the surface of the water so they also are better at penetrating deep tanks. The are among the most expensive light fixture available for the aquarium, so for most hobbyists they would not be the first choice to illuminate refugiums and planted aquariums.

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