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.

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