Light is the energy that drives your plant's metabolisms. Without light, your plants don’t have the energy to take up nutrients, grow, and survive. You won’t get far without gas in your car, and plants will die very quickly without sufficient lighting.

When I was first started learning to grow plants, all we knew to use for lighting was the bulb that came with the aquarium. At some point, someone made the observation that the sun is much brighter than that one lonely bulb in our hoods. Therefore, since plants grow in sunlight (and not in our tanks), maybe what was missing was sufficient light. Thus, the aquatic plant revolution started in the United States. People began adding more light to their tanks, and the plants started to grow. Finally some success!

Soon after people increased their light, they decided maybe it wasn't such intense light that was needed but the proper spectrum of light. Immediately, special color rendition Vita-Lite and Triton bulbs became must-haves for everyone trying to grow aquatic plants. The plants looked better, but those bulbs were very expensive. Over time, the consensus turned back to intensity being the main factor in successful plant growth.

Today, it is generally recognized that plants will grow in a variety of spectrums as long as there is enough light present. The taste of the observer is what mostly determines the appropriate color of light needed. Plants use light in the blue and red areas of the spectrum, but blue and red don't make for a pretty aquarium. Various broad spectrum "daylight" blulbs are the choice of most aquatic gardeners.

The color of a light is usually expressed in degrees Kelvin. A light with a rating of 5500 Kelvin or K is considered to be that which most approximates natural Sunlight. Lights with a lower temperature such as a 4300K are more yellow and orange. Higher Kelvin rated lights, such as a 10,000K are more blue. Plants will grow in the extremes, but again, what matters is how the aquarium looks to you. For people that only have one bulb, they usually pick one somewhere in the mid range such as 6700K. People who have multiple bulbs often mix several different temperatures to bring out the various colors in their plants. As a side note, Kelvin is not the only method of qualifying light spectra, but it is the more commonly employed system by aquatic bulb manufacturers.

The amount of light you have is usually measured in watts per gallon. Simply add up the wattage from each of your bulbs, and divide it by the number of gallons your tank holds. While this system is presently the standard, it is generally considered a poor method of quantifying the light you have. Some good alternatives have been proposed, however at this time no better system is recognized or universally accepted.

Actually, how much light you need is a subject of intense and sometimes livid debate. For years people have used the measurement of 2-4 watts/gallon. These days people grow plants with less and many people have much more. Generally about three watts/gallon or less is a low light tank and more than three watts/gallon is a high light tank. There is definitely no broad consensus in this subject.

Other things that factor into the amount of light you need are how deep your tank is and what plants you want to grow. Very deep aquariums may need more light so that the illumination will penetrate down to the gravel. Also, different plants have different light requirements. (discussed later)

How long you should leave your lights on (or photoperiod) usually depends on the amount of light you have and algae. For most people it is about 8-12 hours daily. If you have low light, you may need a longer photoperiod, and if you are getting algae, you can reduce your photoperiod to see if that helps.

Often plants are classified as low or high light plants when in truth most are neither. Many “low light” plants will grow and flourish in very high light. The error is that these plants don’t usually prefer the lower light, they just tolerate it better than high light plants. Correspondingly, “high light” plants won’t always die in lower light. They usually just grow slower and more subdued. For example, you might try to create a foreground from the plant Glossostigma elantinoides or "Glosso." In high light it grows as a runner with small leaves that tightly hug the gravel creating a lush lawn effect. Glosso is considered a high to very high light plant, but nothing could be more from the truth. It will grow great in mid to lower light. It will probably just grow straight up like a stem plant. Cryptocoryne are known as low light plants when in nature, many grow in direct sunlight. So in summary, the light needed to simply keep a plant alive may be different from that which is necessary for it to flourish and reproduce. Again, plants will survive and grow under a variety of lighting conditions. It is only when you get into the extremes where concerns about high- and low- light loving plants come into consideration. For beginners’ sake, here in the real world, if you stick to a moderate light level you will be able to successfully and beautifully grow about every common plant in the hobby today.

There are three types of lighting commonly used in planted tanks. They are normal output fluorescent (NO), compact fluorescent (CF), and metal halide (MH). They are all good choices for lighting with each having its own advantages and disadvantages. Some factors to consider when looking at lighting are cost, spectra, and how quickly a bulb loses its intensity. Cost and loss of intensity are related because one bulb may be cheaper, but if you have to replace it three times more often than one that is twice as expensive, you are not saving money.

Normal output fluorescent lights come in the best selection of colors and have cheaper options available. However, some tend to lose their intensity relatively quickly and may not be bright enough for some deeper tanks. Compact fluorescents have a strong intensity that is mostly retained throughout the life of the bulb. They can be expensive, but this is offset by their long life. The pins that connect the bulb to the socket are sometimes different in bulbs from different manufacturers. This can be a pain, but it has improved in recent times. Lastly, compact fluorescent lights can get very hot. Metal halides fairly quickly lose their intensity, are expensive, can get dangerously hot, and do not have as wide a selection of colors to choose from. However, the beautiful pinpoint beam they project causes a tank to sparkle and makes you forget about their negative attributes. Many chose MH lighting solely on the look and ignore the negatives. In a visual hobby, what other reason do you need?

High output (HO), very high output (VHO), mercury vapor, and high pressure sodium lights are also used in planted tanks but not as often. T-5 bulbs are the newest type available, but so far there is only limited experience with their use. Lastly, LED lighting is being discussed for the future, but so far I know of no available fixtures using this technology. Stay tunned.

The best lighting is only as good as it's reflector. Reflectors range from the useless white piece of plastic in the top of the hood, to super shiny polished aluminum. Obviously the more mirror-like the reflector is, the better. Reflector evaluation also needs to involve shape of the light beam reflected. A very tight beam will only illuminate a small area of the tank while the corners are in the dark. Some reflectors scatter light everywhere making it almost as useless as that white plastic piece. The ideal reflector would be very shiny and bent several times as to look like an upside down "U" with the ends spread a bit wide. Confused? Think arch under a very large suspension bridge.

I definitely don’t want to leave the impression that high light is what you need to successfully grow aquatic plants. The truth is quite the opposite. There is at least a minimum of light required for plants to grow and survive. You don't have to have 6 watts/gallon, but usually you need more than that which came in the hood supplied with your tank. Success can be achieved with many levels of lighting.

Again, light is the energy that drives the life processes of your plants. The amount of illumination you have will determine how much fertilizer your plants take up, how much you need to add, and how often you must add it. This is aside from CO₂ which will be absorbed faster in high light, but should be maintained at appropriate levels regardless of how much light you have. The amount of light you have is often the great equalizer that, apart from CO₂, determines how you carry out all the other aspects of growing plants.

Published with permission © 2005 The Author- All Rights Reserved.

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