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The Correct Lighting for Your Aquarium
by Dr. Timothy A. Hovanec

Traditionally, hobbyists rarely gave the choice of lights for their aquarium much thought. This might have been due to there only being a few types of light available in any case. Nowadays, the choice of lighting systems and the lamps to go with them are much greater. This offers the hobbyist a better chance for success but also can increase the confusion on which is the proper choice. The purpose of this month's column is to explain some of the more popular types of lighting systems and the lamps that can go with them.

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As with aquarium filtration, there are many "right" ways to light your aquarium, conversely there are also many wrong ways. However, the techniques outlined here should insure success in almost all cases. The importance of the light system ranges from the purely decorative, meaning that it is not critical to the life of the aquatic organisms in the aquarium, to extremely important for sustaining aquatic life. Soft and hard corals will not survive in an improperly lighted reef system no matter how much money is spent on the latest state-of-the-art filtration system.


As with filtration, lighting has its own jargon. Thus, a few terms need to be introduced and defined so you can start to understand the language of lighting:

Color Number: this is more correctly called color temperature. It refers to the absolute temperature, in degrees Kelvin (¡K), of the light produced. It is important when trying to simulate the color of natural sunlight which is about 5,000 ¡K.

Lumens: a measure of light intensity. It is the radiant energy from the visible portion of the light spectrum hitting a given area (say, meters2) when the surface is the unit distance (in this case, one meter) from the light source.

Lux: a measure of the illumination. It is the illumination from all light sources hitting a surface from a distance of one meter. It is equal to lumens per square meter.

Full Spectrum Light: a light source that emits all the wavelengths of the visible spectrum in proportion to that of natural sunlight. A lamp or bulb labeled full spectrum means that it emits light over the entire visible spectrum with a spectral output similar to that of the sun. Most lamps have technical data sheets that provide a spectral chart which can be used to determine the intensity and color of the lamp.

Intensity: This is also an important term. It is related to the electrical consumption (in watts) of the lamp or bulb and is measured in lumens. The more watts a lamp requires, the greater the light intensity. Most books on aquarium lighting give a formula or guide for determining the total wattage one should have from the lighting system over an aquarium. In some cases, this requires a large number of lamps. If lamps of higher wattageare substituted, the number of lamps can be reduced without sacrificing intensity.

Various levels of intensity are also available in the same length lamp so greater intensity can be generated without the need for additional space for more lamps. Symbols are also used to designate these lamps. Lamps that are High Output are labeled HO. Very High Output lamps are labeled VHO. For instance, a normal 24" lamp is 20 watts. A 24" HO lamp is 40 watts, while a 24" VHO lamp is 75 watts. For 48" lamps, the wattage is 40, 60 and 110 for normal, HO and VHO lamps, respectively. High Output and Very High Output lamps require a special ballast. Do not use them in standard fixtures with a normal ballast. High intensity lamps can be used on deep tanks, reef tanks, and plant aquaria.

Wavelength: This is another term that is important to understand, especially in the context of lamp descriptions such as full spectrum or peak wavelength. These terms refer to the wavelength output of the lamp or bulb. Actinic lights, for example, produce only light at a specific wavelength--420 nm. This peak wavelength value, which produces a very blue light, was chosen because during photosynthesis chlorophyll a absorbs light near this wavelength. To promote photosynthesis in reef coral, actinic lamps are used. Some lamps have two, or even three, peak wavelengths.

Types of Lights

Standard incandescent light bulbs can be used for lighting an aquarium but they are not recommended. They are rather inefficient, so they produce a great deal of heat. This can burn the hobbyist and cause the water temperature to rise. Second, they do not last long and consume a relatively large amount of electricity.

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Fluorescent tubes or lamps are more efficient and do not produce nearly the amount of heat standard light bulbs do. To emit light, fluorescent lamps are filled with a special vapor (usually mercury). The inside of the lamp is then coated with phosphors. Turning the lamp on charges the vapor which emits UV light. The UV light hits the phosphor coating causing the lamp to produce visible light. Special use and different types of lamps are made by changing the coating material. By combining different phosphors or groups of phosphors lamps can be made that emit light with certain peaks wavelengths.

Metal Halide lamps are used increasingly in reef aquariums. They produce a very intense light and have various color temperatures. Metal halides work well but have drawbacks. They are less efficient than fluorescent tubes, get very hot (requiring a fan to get the heat away from the aquarium and hood), and are expensive.

General Lighting Guidelines

A commonly asked question is: how long should the aquarium lights be on each day? Since most of the inhabitants and plants for aquariums come from the tropical regions of the world, it is best to mimic the day-length of this region. Length of daylight varies little seasonally in tropical areas. Generally, the light period is 12 hours with an intense period of 9 to 10 hours. Keeping lights on for over 10 to 12 hours per day is of no practical benefit and can cause algae blooms. It is best to buy an inexpensive timer and automate the light system.

Another common question is how long will the lamps last? Usually lamps should be changed before they actually stop emitting light. The reason is that the color spectrum of a lamp changes as the lamp ages. While the lamp may still light, it does not emit light of the original wavelength. A common problem is for the hobbyist to use a lamp until it no longer radiates light. Lamps should be changed at least once a year, but preferably every 6 to 8 months. If, for seemingly no reason, your aquarium starts to grow algae, think about when was your last lamp change? If it was over 6 to 8 months, consider that the lamp's spectrum may have shifted and the lamp needs changing.

The deeper an aquarium or the more particulate material in the water, the more light that is absorbed and/or scattered, so less reaches the gravel or tank bottom. This is an especially important consideration in plant and reef tanks. Consider adding an extra lamp for each 15" of tank height over 20".

Lights for different tanks set-ups

Fish-only tanks can have a very simple light system. The purpose is to show off the fish and tank setup. The final decision depends upon the individual taste of the hobbyist. For 20 gallon tanks and under, one fluorescent lamp is adequate. From 30 to 55 gallons, two lamps, minimum, should be used. Add an additional lamp for each 20-25 gallons of water capacity. A lamp with a 5,000 ¡K to 6,000 ¡K color temperature is recommended, but many hobbyists prefer lamps which emit more red color as they can show the fish colors better. This is fine, but these types of lamps will not promote the growth of plants.

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Plant tanks require the correct lighting to be successful. The number one reason for lack of success in growing plants in an aquarium has to be the use of the wrong lamp. Plants have two types of chlorophyll, a and b. Chlorophyll a absorbs light at 405 and 640 nm. Chlorophyll b has a peak absorption at 440 and 620 nm. Plant lamps are designed to emit light at the red wavelengths to duplicate the job of the sun. But too much red color can cause aquatic plants to grow tall and thin. For best results, use a daylight (5,000 ¡K) lamp such as an Aquasun, Ultralume 50 or Chroma 50 in combination with an actinic white or actinic day lamp. The actinic day or white lamp is a mixture of 50% actinic (blue light) and 50% daylight. In large or deep aquaria consider using HO or VHO lamps.

To have a successful reef aquarium, adequate light is absolutely required. Reef tanks contain soft and hard corals which harbor zooxanthellae. The zooxanthellae (symbiotic algae) must thrive in order for the coral to live. To do this they need the correct amount of light (intensity) at the right wavelength (peak absorption). Actinic lights provide a concentrated light wavelength that promotes photosynthesis. If only actinic lamps are used, however, the water color in the tank will be very blue which is not visually appealing and the light is not intensive enough for the other processes of the aquarium inhabitants. Therefore a reef tank should have a combination of one actinic lamp and one or two daylight lamps for each 30 gallons of water. The daylight lamp can be either a metal halide bulb or daylight fluorescent lamp (preferably HO or VHO). The color temperature of the bulb or lamp should be 5,000 to 6,000 ¡K.

The other popular type of reef system is to combine metal halide lamps with the fluorescent lamps discussed above. Figure on one metal halide per 25 gallons of water for a really successful reef tank.

Final Words

Generally, not much attention is paid to the lights for a freshwater fish-only aquarium. It seems only a little attention is paid to the lights for tanks where the goal is to grow plants. If you have algae problems, or plants that won't grow, or corals and anemones which waste away after 3 or 4 months, suspect improper lighting as the cause. Discuss the specifics of your tank (size, shape, total gallons, etc.) with your favorite store clerk. Listen carefully to the suggestions he/she makes, do they make sense? The right light can mean the difference between success and failure of your aquarium.

© 1998 Dr. Timothy A. Hovanec
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