Lighting

Most aquarium fish come from tropical regions, where there’s always about 12 hours of sunlight a day throughout the year. These fish don’t do well when exposed to natural light in temperate regions, where the length of a day varies. It is best to duplicate a fish’s tropical environment by providing 12 hours of controlled lighting every day. Do this by installing artificial lights controlled by a timer.

How Much Light Does an Aquarium Need?
Water is a great absorber of light, a fact that sometimes makes it easy for aquarium hobbyists to underestimate the lighting requirements in their tanks. A good rule of thumb is to provide 2 watts of lamp power for every gallon of aquarium water, so a 50-gallon tank is illuminated by a 100-watt bulb.

Aquariums with live plants will need more lighting than a fish-only tank. Although fish will do well if exposed only to fluorescent ("daylight") lamps, live plants will require a wider spectrum of colors in their lighting. (See below.)

The "Nature" of Aquarium Lighting
Light falls within a broad spectrum of wavelengths, with each wavelength producing a different color. In nature, different wavelengths are dominant in sunlight at different times of the day. The "warm" sunlight of morning and evening contains a lot of red wavelengths. During the middle of the day, "cold" blue tones are more prevalent in sunlight.

Different light tones benefit aquatic plants in different ways. Red tones stimulate vertical growth, while blue light encourages strong compact plant growth. To provide plants with a "balanced" lighting diet, good aquarium lighting should produce a mix of colors. This can be done by illuminating aquariums with "full spectrum" lamps that provide an all-in-one lighting blend, or by installing different lamps that give off light dominated by different colors. (See "Aquarium Lighting Options.")

Providing plenty of blue wave light to an aquarium is important to the health of corals, anemones and other soft-bodied marine invertebrates. Many other invertebrates such as lobster, crabs and sponges prefer less intense lighting. Owners of these invertebrates should provide shaded areas in their tanks.

Balanced lighting is not important to the health of fish, the way it is to plants and invertebrates, but it can help enhance the appearance of freshwater and marine fish as well as their tank surroundings.

Measuring Aquarium Light
There are different ways to measure and describe light:

     - Lumens -- the intensity of the light produced by a lamp.
     - Lux -- the amount of lumens per square meter.
     - Kelvin Degrees -- measures the degree of color in light. This measurement is written as ° K.
     - Color Rendering Index (CRI) -- measures the similarity of artificial light to the sunlight’s Kelvin        Degree reading at noon.

Fluorescents are the most common type of lighting, especially in freshwater tanks. Most aquarium hoods come with fixtures for one or more fluorescent tubes. One of the big advantages of fluorescent tubes is that they give off very little heat, even though they produce a lot of light. Fluorescent tubes are also inexpensive, easy to install and reliable.

The disadvantage of fluorescent tubes is that their light does not penetrate very deeply into water. Those with tanks deeper than 25 inches will not be able to shed much illumination on bottom water using only tubes. They will have to rely on "ambient light" from the area around the tank, or on one of the aquarium spotlamps described below. Installing a reflector over a fluorescent tube will increase the intensity of its light. Many fluorescent tubes also come with built-in reflectors.

Power Compacts are recently developed fluorescent lighting units that produce far more light than traditional fluorescent tubes. Although they are more expensive than tubes, power compacts give off about twice as much light per watt, and can go longer before being replaced.

Fluorescent lights are available in a wide variety of options to produce different colored light. Here are some examples:

     - Natural Daylight White -- A good choice for the fish-only tank, these lamps produce a light        that is easy to look at, but offers little in the way of nourishing plants.
     - Actinic Light -- These lamps produce light with a very heavy concentration in the blue        wavelengths. This makes the light especially good for corals, anemones and other invertebrates,        as well as some beneficial marine algae. Used mainly in marine tanks, this light gives water a        pleasant blue cast, especially when used with other lamps.
     - Combination Actinic -- A popular option is to combine blue actinic light with natural daylight.        Many models offer a "50/50" combination built into the same fluorescent tube.
     - Full Spectrum -- Powerful lamps that simulate natural sunlight by producing all colors needed        to stimulate plant growth.
     - Growth Lights -- Rich in violet wavelengths, these lights stimulate plant growth.
     - Metal Halide Spotlamps
       Used primarily in marine tanks, metal halide units produce a high-intensity light with a wide color        spectrum. Metal halide is especially good for coral and other reef organisms, because it        produces large quantities of blue wave light. The intensity of metal halide spotlamps ensures that        their light will penetrate deep marine tanks and coral systems.

       There are two main drawbacks to metal halide lights. They are relatively costly, and they emit a        great deal of heat. In the warm weather months, the heat from a metal halide lamp can raise tank        temperatures to dangerous levels. Plan for this by increasing room ventilation, installing fans by        the lamps or adjusting heater thermostats. (See "Heaters and Temperature Control.")

Mercury Vapor Lamps
These lamps resemble metal halide units in appearance. Mercury vapor lamps are strong in the green, yellow and orange wavelengths, which creates a good visual impression in the tank. However, mercury vapor is weak in the blue wavelength light that is needed by many invertebrates, so those who use mercury vapor should complement it with a blue actinic fluorescent. Mercury vapor is only for experienced aquarium owners with a specialized interest in tank lighting.

Installation and Replacement Notes
Placing a glass cover between the lighting unit and the tank water surface will keep lamps from becoming wet and short circuiting. To reduce the heating effect on water and lower the risk of lamps becoming wet, keep metal halide and mercury vapor spotlamps at least 8 inches above the tank water surface.

Never line the aquarium hood cover with aluminum foil to increase the reflectivity of fluorescent lamps. This will block ventilation holes that are needed for the tank water to "breathe" (exchange carbon dioxide for oxygen).

Positioning & Sizing Lights
A fish orients itself by the gravity "pulling" from below, and the light entering from above the water’s surface. This is why aquarium lights should always be at the top of the tank. Positioning a light at the side of a tank will cause fish to become disoriented and swim at an angle. Whenever possible, it’s a good idea to aim lights slightly to the rear of the tank, since this will cast shadows behind the fish, and away from the viewer.

A fluorescent tube should be close to the same length as the tank itself. Generally, this means that a 20-gallon rectangular tank should take 24-inch fluorescent tubes. Each mercury vapor or metal halide lamp should cover about 2 square feet of tank space.