If you are setting up an aquarium in your home or office, there are a number of factors you need to consider. First, what type of ecosystem do you wish to create: freshwater, saltwater, a reef environment, a river environment? The size of your tank, and the equipment you’ll need to outfit your tank, all depend on what you wish to stock the tank with. (If you’re a beginner, a smaller tank is generally more suitable.) But regardless of what your eventual aquarium environment will look like, you’ll need to light it.

Since your aquarium will most likely be indoors, you need lighting to be able to view your fish and plant life; overhead or ambient lighting that is already present in the room where you’ll be keeping your aquarium is rarely sufficient. But, most important, your plant life and much marine life (such as coral and anemones) require light for photosynthesis. Lighting also influences fish behavior and affects the overall well-being of your aquarium environment.

Aquarium lighting fixtures are generally mounted to a hood or canopy that fits over your tank. Obviously, you must have a hood that is suitable for the size of your tank, and you should discuss your options with your dealer when you first purchase your tank. Most aquarium lighting is fluorescent (normal output or compact) or high-intensity metal halide, and you will need to determine what you will need before purchasing a tank and hood, to ensure that they can accommodate the style of lighting, and size and configuration of light bulb, that you will require.

As a general formula, you will need 2 watts of lighting for every gallon of water in your tank; a 60-gallon tank would then require 120 watts of lighting power. However, if you have dense plant life with higher requirements, then you’ll need at least double that -- figure on 5 watts per gallon -- and reef tanks need even more. Bear in mind that lighting is a complex subject, and the watts-per-gallon guideline is only the beginning of the discussion; many other variables can come into play, especially as lighting systems become more advanced. The output from a 60-watt metal halide bulb will differ significantly from the output from a 60-watt floodlight, for instance. Other variables you may wish to consider include lumens per watt, PAR (photosynthetic active radiation), PUR (photosynthetic useable radiation), and even the space the bulbs will occupy in the hood; do some research on the Internet and discuss the issue with your dealer or with an experienced hobbyist.

You want your lights to bring out the vibrant coloration of fish, coral, plants, and other life forms you may have in your aquarium. Light can have a range of visual quality with respect to how colors are rendered to our eyes, and this is measured by the CRI (color rendering index) of a particular type of light bulb. The CRI of a bulb is based on a scale of 1 to 100, with 100 indicating how a lit object will appear in natural daylight conditions. Full-spectrum bulbs -- bulbs that emit all the wavelengths of visible light -- approximate natural light most closely, and thus have high CRI values. However, you may wish to enhance certain colors by using color-enhancing bulbs, which emit light from the “warmer” end of the color spectrum and accent reds and yellows. Many enthusiasts combine full-spectrum bulbs with color-enhancing bulbs.

Another commonly used measurement is a bulb’s color temperature, measured by its Kelvin rating (K-rating); the K-rating describes the temperature (in degrees Kelvin) and corresponding range of colors of a light source. The progression of colors from the lower end of the Kelvin scale begins with reds and oranges, to yellows, greens, blues, and indigos, on to violet at the higher end. Oddly, the colors highlighted by bulbs with lower K-ratings (reds and yellows) are considered ”warmer,” whereas the blues and violets highlighted by bulbs with higher K-ratings are considered “cooler.”

Sunlight at midday has a K-rating of 5,500 degrees Kelvin and contains a blend of all the colors in the spectrum; therefore, a 5,500 Kelvin bulb is a full-spectrum bulb. Bulbs with a lower K-rating give off reddish light, and bulbs with higher K-ratings emit bluish light. Freshwater aquariums generally do better with full-spectrum bulbs, perhaps complemented by some warmer color-enhancing bulbs. Saltwater aquariums, particularly reef aquariums, usually require higher K-ratings, at least 10,000 degrees Kelvin. Corals and invertebrates have naturally adapted to bluer light and will thrive in a cool-light environment.

As for the functional purpose of aquarium lighting, your lighting will be the primary, and often the only, source of light for your plants, corals, and other photosynthetic organisms. To ensure that this life-sustaining process proceeds smoothly, you should mainly be concerned with the intensity of your bulbs; total wattage is the primary measure of light intensity. Freshwater planted aquariums require 2-5 watts per gallon, but saltwater reef aquariums will need more, as much as 8 watts per gallon.

Many reef aquariums are lit with bulbs producing “actinic” light; these bulbs are high intensity and will promote photosynthesis in your coral as well as your reef plant life. However, because actinic bulbs produce light that is strongly blue, they need to be balanced with warmer light, or with full-spectrum bulbs. A “50/50 lamp” combines full-spectrum light (usually 6,000 degrees Kelvin) with actinic light in a single bulb, and would thus be a solution if you only have a single fixture in your hood.

Once you have decided on your lighting, be sure to install the lights on a timer. Most aquatic environments do best with 10-12 hours of light each day, approximating natural conditions; fish need “down time” the same as humans! If you are prone to forgetting to turn your aquarium lights on and off on a daily basis, a timer will do the job for you.