Let There Be Light!

Copyright 2000 by George and Karla Booth

Note: In case you get bored before you get to the end of this article, I would like to recommend Aquarium Hobbyist Supply as a most excellent source of compact fluorescent lighting kits and supplies. The owner, Kim, has gone out of his way to develop a superb series of lighting kits to suit any need. If you are retrofitting a standard canopy or building a custom hood, AH Supply has what you need. Joe Bob sez, "Two thumbs up!"

Humans have been using artificial lighting for a very long time. It is second nature for us to walk into a darkened room, feel for a light switch and light our way. Aquarists who have fish-only setups have been using lighted hoods for many years and don't spend much time struggling with the concept. Any bulb that fits in the hood and has a pleasing spectrum is suitable. Many of the aquarium specific bulbs are enhanced in the blue and red spectrum to bring out those colors in the fish.

On the other hand, if you visit any of the Internet sites devoted to planted aquaria you will quickly realize that lighting is a very popular topic. You will also notice that the topic seems to be fraught with misinformation, myths, wives tales and amazing claims by lighting suppliers. So why is it that freshwater plant enthusiasts end up thinking that proper lighting is "a puzzle wrapped in an enigma"?

One of the most common questions asked is "What's the best light for plants?" This question seems to get the most varied answers simply because "best" can be defined in so many ways. Are you trying to optimize intensity? Are you looking for the best spectrum for growth? Do you desire a natural appearance? Do you want to take prize-winning photos? Are energy costs critical? Is the initial expense a stumbling block? Do you have a closed hood or open top? How deep is your tank?

If you don't know much about lighting terminology and technology, these can be difficult questions to answer. But, armed with a little knowledge, you can easily optimize your lighting setup to suit the needs of your tank inhabitants and your personal preferences. Lighting can seem complicated, especially when an expert starts spouting impressive sounding terms, but it is really very straightforward.

The are many benefits associated with optimum lighting. If you are growing live plants, proper lighting is critical to good growth and plant health. If you are interested in photographing your aquariums, whether for contests or publication, good full spectrum lighting will make your job a lot easier. Finally, with the correct lighting, you will find that your aquarium, whether it's fish-only or fully planted, will be displayed at its best. Professionals in the "display" business, such as museums and art galleries, know that proper lighting is the number one way to showcase their collections. Our fish and plants deserve no less!

Lighting Terminology

Understanding the meaning of a few key terms is the first step in the search for success with lighting.


Bulbs come in various wattage ratings. This describes how much electrical power a bulb uses and does not describe how much light it generates. Different bulb technologies will produce differing amounts of light per watt. More technically, different types of bulbs are more efficient at converting electrical energy into light energy. For example, a typical fluorescent bulb is four times as bright as the same wattage incandescent bulb and a metal halide bulb is two times as bright as the same wattage fluorescent bulb. Even within a technology, light energy per watt will vary. All normal output 40-watt fluorescent bulbs draw 430 ma of current but vary greatly in how efficiently they convert the energy from the resulting internal arc into light.

Unfortunately for the aquarist, wattage is the simplest and most common way to specify how much light is needed even though it is not an accurate measure. It is very common to see "use 2-3 watts per gallon of fluorescent light for plants". This is a very coarse rule because it doesn't take into account the efficiency of the bulbs, the type of reflector, the form factor of the tank or the type of plants being grown. But it is a good starting point because all bulbs are rated in watts. It's the least common denominator of light bulb specifications and it may be satisfactory for a lot of plant enthusiasts.


Lumens are is the total amount of light that a bulb is capable of generating. This information should be readily available, either on the bulb package or from manufacturer's data sheets. It may take some digging to find it though. There are two values usually quoted for fluorescent bulbs: initial lumens and design lumens. Initial lumens describe how much light it produces when first turned on. Design lumens describe how much light it will produce for a much longer term. After an initial 20% drop in brightness, the light output will slowly decrease over the lifetime of the bulb.

Lumens are somewhat misleading since they are geared towards what the human eye perceives. Human eye response is very sensitive to green light and less sensitive to the red and blue ends of the visible spectrum. Conversely, plants are more sensitive to red and blue light than green. A bulb heavily weighted to produce light in the green spectrum bands will be perceived as much brighter to the eye than bulbs that concentrate energy in other bands but plants will not be able to use much of the energy. A "Cool White" fluorescent bulb (designed to take advantage of this phenomenon) looks bright to us and has a high lumen rating but won't grow plants as well as other types of bulbs. The bottom line is that even if a bulb has a high lumen rating, it might not be suitable for aquarium lighting.

The lumen rating is a better way to specify lighting than watts since it describes how much light is available. But it still falls short in that all the lumens might not reach the plants and it doesn't describe the energy bands that are generated. A bulb might have lots of lumens but much of the light might be lost because of a poor reflector design or the light might be in a part of the spectrum where plants have poor response.


Lux is a measure of the actual intensity of the light falling on a specified area and is a much better way to describe lighting requirements. Lux is defined as lumens per square meter. Unfortunately, since lux depends on the how the light gets from the bulb to the area, the manufacturer can't specify it - it has to be measured by the aquarist. Hobbyist luxmeters are available for under $150 and are invaluable for the serious light cognoscente. Also note that since lux are defined in terms of lumens, they also suffer from a bias towards human eye response.


This is the Color Rendering Index and describes how closely a light source renders colors compared to "standard" sunlight as observed by a "standard" human eye. A perfect source would score 100. Full spectrum bulbs rate in the 90s. CRI is usually specified for higher quality fluorescent bulbs but does not seem to be a valuable measure for bulbs used in planted tanks. Bulbs with a high CRI may not necessarily have the right spectrum for plants and may not be bright enough for good plant growth.

Kelvin Temperature (K) or Color Temperature

This describes the overall color of the light but not the spectral components. If a light source is stated as 5000K, it is the same color as a radiating black body (a physics term) heated to that temperature. Incandescent bulbs are around 2700K (reddish), "daylight" is roughly 5500K and higher color temperatures look more blue. In an aquarium setting, daylight or 5500K bulbs may look a little yellow while 6500K bulbs have a pleasing white light.


This describes the wavelengths of light that make up the light source. Visible light is a continuous band of colors ranging from violet to red (wavelengths of 380 nanometers to 700 nanometers). Sunlight and incandescent light are composed of all visible wavelengths. Fluorescent and metal halide bulbs emit only a few wavelengths ("spikes"), depending on the phosphors and rare earths used. Most fluorescent bulbs emit two strong spikes to generate a particular color temperature. A newer type of bulb called "tri-phosphor" adds a rare-earth phosphor to generate a wider spectrum of light - usually three spikes - and is strong in the green spectrum. Tri-phosphor bulbs are more expensive than other bulbs but generally give a pleasing appearance to the aquarium and are well suited for good plant growth.


Photosynthetic Active Radiation is the true measure of how plants respond to light. Just as lumens are keyed to human eye response, PAR is keyed to photosynthetic response. If bulbs were rated in PAR, it would be easy to select the best bulb for plant growth (but not necessarily for good appearance). Unfortunately, PAR meters are expensive and are not generally available to hobbyists and few if any bulb manufacturers supply PAR specifications. So, until someone sees the light, we are stuck with selecting the best bulb based on the other ratings.

Lighting Technology

Aquarists will typically use one of three types of lighting: incandescent, fluorescent or high intensity discharge. Sometimes combinations of lighting technology are used for special purposes. Also, some plant enthusiasts will augment artificial light with sunlight but, of course, sunlight is hard to control.


Incandescent lights were the original source of light for indoor aquariums. Although cheap and plentiful, they have serious drawbacks. First, they aren't very efficient - they generate a lot of heat for the lumens they produce. This fact alone makes them useful for only the smallest tanks. Secondly, although they are "full spectrum" in the sense that they generate all bands of visible light, their color temperature is only around 2700K. The result will be a reddish appearance and plants will have a sickly yellow color.

"Halogen bulbs" are a newer form of incandescent lighting. These produce a whiter light than normal incandescent bulbs and can be obtained in small sizes for aquariums under 10 gallons. They still have the drawback of inefficiency and the light will still look a little yellow compared to better fluorescent lighting. One option is to use this type of lighting to highlight a specific plant or create a special effect. Amazon sword plants seem to do well when some incandescent light is available.


Fluorescent is the most commonly used lighting for aquariums because of its efficiency, low cost and form factor (long and thin). There are many choices in fluorescent lights, which has led to a lot of confusion among aquarists.

There are three categories of bulbs commonly in use today. The most familiar is the standard fluorescent bulb. These bulbs are nominally rated at 10 watts per foot - 48" 40-watt, 36" 30-watt, etc. They are often classified by their diameter in eighths of an inch. For example, a "T12" bulb is 1 1/2 inches across. Forty-watt bulbs usually come in T12 and sometimes T10 sizes but lower wattage bulbs can be anywhere from 1/2" in diameter on up. Standard bulbs can be further classified as normal output (NO), high output (HO) and very high output (VHO). NO bulbs are by far the most common and are used in most commercial aquarium hoods. The higher output bulbs draw more current and produce more light in the same space but you need special ballasts to drive them and you may need fans to cool them. One advantage of the standard NO bulbs are the wide variety available - almost any color temperature and spectrum can be obtained.

Standard NO bulbs can be run with older-style "tar" ballasts (essentially special purpose transformers) or with more modern electronic ballasts. Higher output bulbs require specially designed electronic ballasts. An electronic ballast is preferred over a tar ballast because they are much more energy efficient (producing far less heat) and drive the bulb with a higher frequency signal (producing more lumens).

You may find inexpensive shop lights for around $10 that claim "energy savings" and "electronic ballasts". You should avoid these - they save energy simply by supplying less power to the bulb and, as a consequence, the bulb will produce less lumens than it should. Also, they use a simple capacitor-inductor circuit instead of a ballast that can reduce the lifetime of your bulbs.

A newer style of bulb is a true "energy saving" fluorescent bulb. It is commonly seen in a 48" long, 32-watt T8 bulb. Even though they have a lower wattage rating, they produce the same or more lumens than standard 40-watt bulbs. The bulbs themselves are more efficient and use electronic ballasts especially designed for them. This combination will lower your overall energy costs. Currently there are fewer choices available in T8 bulbs but that should change in the future - energy saving bulbs are mandated to replace standard bulbs because of their higher efficiency and manufacturers will be developing complete lines of these bulbs.

The newest fluorescent bulb in the aquarium market is the "compact fluorescent" (CF) bulb. They are so named because the bulb is long and skinny and usually is folded in half. This allows you to have more light per inch - especially valuable for aquariums less than 48" long. CF bulbs are designed to use efficient electronic ballasts. Furthermore, because the tube is skinny, a very efficient reflector can be designed for them, increasing the amount of lux produced. There are fewer color choices available in CF bulbs but there is a large enough selection to satisfy the needs of most aquarists.

A typical CF bulb is a 36-watt bulb that is only 17" long and 2" wide! One can easily fit four of these over a standard 75-gallon aquarium (48" long by 18" wide), giving you 2 watts per gallon. This may seem like the low end of the "2-3 watts per gallon" rule, but the CF bulbs produce more lumens per watt than standard fluorescent bulbs. Combined with an efficient reflector, this combination will produce enough light to grow even the most demanding plants.

The photo shows four 55-watt CF bulbs (22" long) installed in a hood for a 100 gallon tank (60" x 18"). Note that there is still room for a small 13-watt red CF "night light" in the middle rear. These lights were obtained from a company that sells a full line of CF lights for aquarists, Aquarium Hobbyist Supply (http://www.ahsupply.com).

High Intensity Discharge

To get the maximum light in the least space, many aquarists turn to high intensity discharge (HID) technology. The systems used on aquariums are usually mercury vapor (MV) or metal halide (MH) lights. These lights are related to the orange sodium vapor lights seen in parking lots. All the systems use a small bulb containing a metal (sodium, mercury or halides) that is vaporized by an arc in the bulb. The vaporized metal then gives off the light we see. The small bulb is usually enclosed on a larger bulb that offers UV protection. These bulbs are very efficient and give off intense light - typical bulbs are rated at 175-watts and 250-watts. These bulbs also require special ballasts to drive them. They are very popular in reef setups and hydroponics culture.

MV and MH lights are ideal for suspended hoods over large open-top aquariums. Typical applications will have two 175-watt bulbs over a 48" long tank or three 175-watt bulbs over 72" and 96" long tanks. Bulbs are available in color temperatures of 4300K and 5500K (suitable for planted tanks) and higher color temperatures more suited for reef tanks. One very nice visual side effect of this type of light is that it is close to a "point source" and will cast shadows in the tank just like real sunlight.


Without getting too involved with measurements and esoteric specifications, the old "2-3 watts per gallon" rule still holds fairly well for fluorescent bulbs and typical tank dimensions. The type of plants you grow and how densely the tank is planted will determine how efficient your light system needs to be.

There are many directions you can take to improve your current lighting system. If you already have a good quality system, you might consider the age of the bulbs. Fluorescent and HID bulbs degrade with age and may lose up to 50% of their initial intensity after a year of operation. While the bulbs may have been bright enough when they were new, they may be sub-optimal now. MH bulbs will also see a spectrum shift with age, drifting toward a lower color temperature. If you have a multi-bulb set up you should change a single bulb periodically to avoid sudden changes in light intensity. If you aren't using tri-phosphor bulbs, you might consider investing in better quality bulbs as you change them.

If your aquarium hood does not have a reflector, you can improve the amount of light reaching the water by either painting the inside white or lining it with silver Mylar film available at hydroponics stores. If you can fabricate a metal reflector in a W-shape or a parabolic shape, so much the better. But keep in mind that standard T12 fluorescent bulbs are fat and will block a lot of light being reflected ("restrike"). Thinner bulbs like T8 or CF permit a much better reflector.

If you still have older tar ballasts, investing in electronic ballasts will improve the light output and reduce your electricity bills. If you are changing ballasts, you may want to consider the energy efficient T8 bulbs as well.

If you are starting out with a commercial aquarium light hood, you may find that it is insufficient no matter how you upgrade the existing lights. A 55-gallon tank may come equipped with two 20-watt bulbs. There is no way you can enhance the output of 20-watt bulbs to provide even 2 watts per gallon! The best course of action is to consider retrofitting the hood with CF bulbs. With the proper CF retrofit kit, even a small 10-gallon aquarium can have enough light to grow the most light-loving plants.

We hope that this article has made it easier for you to determine if your lighting is as good as it can be. Knowing the terminology and options available should allow you to intelligently sort through ads and data sheets to find the system components that are right for you.

Next time you lift your hood and are blinded by the light, ask yourself what your plants may think. To you, the light is blinding; to the plants, light is life itself. Carefully inspect your bulbs, ballasts and reflectors to see if any changes are needed.