Lighting in a Planted Aquarium

There are a lot of options available for lighting the planted tank. That”s the good news. The bad news is that it can be very confusing to sort through them all and decide what will work best in any particular application.

The best place to start is by considering the following parameters that are important when choosing lights for a planted tank.

  1. Intensity of light is more important than spectrum

  2. Aquatic plants use light in the red and blue areas of the spectrum most efficiently. They are less efficient at using green and yellow light, but they are also capable of using even this part of the spectrum as long as the intensity is adequate.

  3. Red light encourages long, “leggy” growth, while blue light encourages compact, “bushy” growth.

  4. Algae is much better at using poor-quality light than higher plants are, so strong light in the wrong spectrum can encourage algae problems, particularly in a tank with nutrient imbalances of one sort or another.

I think it’s safe to say that here in the U.S. the majority of people use fluorescent lighting on their freshwater aquariums. Other types of lighting occasionally seen are incandescent, mercury vapor and, more and more often, metal halides. As a very general rule of thumb — with the exception of incandescent lighting, where much higher wattages are needed to produce the same amount of light — plan on using about of 2 to 3 watts per gallon for good growth in a planted tank up to about 20 inches deep. More light may be necessary for a deeper tank.

Recently, in the typical American thinking of “more is better,” I’ve come across people using upwards of 5 watts per gallon on their planted tanks. While it is possible to manage such a system, it is more work. I have yet to meet a “high intensity” plant that could not be grown well in a tank of normal depth at around 3 watts per gallon. If the plant is not growing at 3 watts per gallon, look to other causes for the failure first.


Incandescent light is not particularly suitable for aquarium use. These bulbs put out a much higher percentage of heat to light than other types of lighting. They are expensive to run, burn out quickly and tend toward the red end of the spectrum — which, as we’ve discussed, produces leggy, spindly growth in aquarium plants.

Mercury Vapor

Mercury vapor lamps, from what I understand, are a viable option for aquarium lighting in Europe, but the proper spectrum bulbs have not been made readily available on the U.S. market. For this reason, I have little experience with them.

Metal Halide

Metal halide lighting is a very popular choice with reef tank owners. These lamps give off a tremendous amount of light. While they do get much hotter to the touch than fluorescent tubes, they are putting out a lot more light at the same time. The percentage of light to heat is actually a little better than the same wattage of normal output fluorescent bulbs.

Metal halides are certainly a viable option for a planted tank, and are the lighting of choice for those who want an open topped tank or for those with very tall or oddly shaped tanks. As a point source type of lighting, they have one strong advantage over all fluorescents — they catch and reflect any slight movement of the water surface. This gives a sparkly, sun dappled effect to the tank that is remarkably similar to a body of water lit by natural sunlight.

The heat for metal halides can be handled in one of two ways. If an open topped tank is desired, the lamps can be suspended pendant-style over the tank. If you prefer a closed tank, they can be mounted inside a hood, with small fans and ventilation openings to prevent a buildup of heat. In either case, the ballasts are heavy, and produce heat, so they should be mounted somewhere off of the tank.

Metal halides are available in a wide range of Kelvin (K) ratings. While to some extent the brilliance of these lamps offsets deficiencies in spectrum, most aquatic gardeners prefer those with a rating in the vicinity of 5000 to 6500 K. This is a color that closely mimics natural midday sunlight. There is also a range of wattage sizes available. Be sure to purchase the lamps that are best suited to the size of your tank — both in terms of wattage and the area that must be lit — because metal halides are much more “focused” light than fluorescents. For instance, over a 4-foot long tank, two evenly spaced smaller bulbs will give much more even lighting than a single, larger wattage bulb suspended over the middle of the tank.

Metal halides can be expensive to purchase initially, especially if you buy new fixtures meant for the aquarium trade. If you are handy, however, used fixtures are often available at a much lower cost, and can be retrofitted for aquarium use. The initial expense of metal halides can, to some extent, be recouped because of the fairly long useable life of the bulbs. From an electricity standpoint, for the amount of light they produce they are equivalent or cheaper to run than other forms of lighting.

The fluorescent tube has become the aquarium industry “standard.” Unfortunately, the single tube that comes with the average aquarium is not adequate for a planted tank. But, by adding strip lights to the top of your tank or by building a custom hood to hold multiple tubes, fluorescents can give you very adequate lighting.

High Output and Very High Output Fluorescents

Some people choose to use high output (HO) or very high output (VHO) fluorescents, and these can give lighting that rivals metal halides in intensity. These tubes give out more light in the same amount of space than normal output tubes, so they can be very useful on moderately deep tanks with limited surface area. They must be run on special ballasts for full efficiency and use special end caps as well. Initial and replacement costs for tubes can be high, and the tubes degrade quite quickly. In terms of efficiency, they are similar to normal output fluorescents, and slightly less efficient than metal halides and energy efficient T-8 fluorescents.

Normal Output T-12 Fluorescents

I look for several things in a fluorescent tube for a planted tank. High lumen rating, full spectrum, a color temperature of between 5000 and 6500 K, a color rendering index (CRI) as high as possible and low cost. Very few tubes measure up in all areas.

Remember that the Kelvin rating does not indicate spectral distribution. It indicates the color of the light that a particular tube will appear to exhibit to the human eye. It has more to do with eye appeal than plant growth. Tubes between 5000 and 6500 K come the closest to natural midday sunlight. Ask to see a spectral distribution chart for any new tube you are considering. It should be shaped like an “M,” with a spike around 5000 K. If the middle of the “M” is shallow, it means they are filling in with light that humans find most attractive. That’s okay, but you want to make sure that both the red and blue parts of the spectrum are well represented, because these are the areas most needed for plant growth.

Next look at lumen output, as well as the drop off over the life of the tube. This is where there is often a big difference between cheaper tubes and more expensive ones. Only you can decide if you’ll save money by changing the tubes less often.

It is possible to “mix and match” tubes in multiple tube setups and have very good results. Some people who are not after optimal plant growth, or who run enough tubes that intensity makes up for any lack in spectral output, find that a combination of “warm white” and “cool white” is adequate. Most aquatic gardeners, however, prefer a full-spectrum tube or a combination of full-spectrum and “daylight” tubes. Actinic tubes are not a suitable spectrum for the planted tank. They tend to encourage algae growth more than higher plant growth. Tri-phosphor bulbs rate the best in all areas but price. Still, if your lighting is marginal, you’ll get better results from these tubes. Also, all fluorescent tubes have a longer life span and degrade less when run on an electronic ballast as opposed to the “tar” ballasts commonly found in shop lights and cheaper aquarium fixtures. You can also extend the life of your bulbs by properly ventilating your hood. Fluorescent tubes perform best if not allowed to overheat.

Whether you are using normal output, high output or very high output tubes, be sure to factor in the cost of changing tubes at least yearly. For top output and the most even lighting, the tubes should be changed every six months on a rotating schedule. In most situations changing tubes on a rotating schedule so that no tube is more than 12 months old is perfectly adequate.

Energy-efficient T-8 lighting

There is a new type of fluorescent lighting that has great potential for use in planted tanks. In fact, before too long, these lights will become the “standard” as higher energy efficiency requirements are imposed on the industry. These are energy-efficient T-8 tubes run on electronic ballasts. They are made with the same rare earth phosphors used in the best full-spectrum T-12 (normal) tubes.

I have been using only energy-efficient T-8 tubes for several years now. Here is a comparison between these lights and a standard 4-foot, two tube shop light (in both cases I am talking about a 5000 K, full-spectrum tube with a spectral curve similar to a Vitalite):

A standard shop light with two tubes uses 94 watts — 40 watts for each tube and 14 for the ballast. It produces 5000 lumens. Each tube (assuming Vitalites, one of the least expensive full-spectrum tubes) costs at least $13, and tubes fall off to about one-half the rated lumens by the one year mark.

A two-tube fixture (using GE’s SPX 50 T-8 tubes with an electronic ballast) runs at 60 watts (32 watts per tube and the ballast runs at 95 percent, which accounts for the additional 4 watts). It produces 6000 lumens. Each tube costs under $10. These tubes drop about 5 percent in lumens in the first 100 hours of operation, but the rated lumens are after that period. The drop off over the remaining life of the tube (average failure time is 20,000 hours) is only to about 80 percent.

It is fairly easy to retrofit a two-tube strip light with a new ballast, and the end caps are the same. The ballasts cost around $40 for a two-tube ballast, but you can also get ballasts for four tubes (a two-tube ballast will also run a single tube if you want).

The one catch to using this type of lighting is that it is available mainly for commercial applications. Even a year ago you couldn’t walk into Walmart or even Home Depot and expect to find them. Now I have heard reports form various areas of the country that these lights are even showing up in these home improvement-type stores. Depending on where you live, however, you may still have to go to a local lighting distributor who provides lighting to contractors for commercial installation. One web source that I know of is Grainger’s.

Regardless of the type of lighting you choose, there are also some guidelines for photoperiod that should be followed. Most of the plants used in aquariums are from tropical or subtropical areas of the world. These areas do not have the great variations in day length we experience in the temperate regions. For this reason, we should try to duplicate a tropical photoperiod of 10 to 14 hours of light daily. People using very strong lights, like metal halides, often find that the lower end of this light period is sufficient. Those with less bright lighting may want to use a longer period. However, in no case should the photoperiod be extended much beyond 14 hours. At this point, whether the lights are on or not, higher plants stop photosynthesizing efficiently, while algae is quite capable of carrying on.

Finally, there are a couple of simple ways to reduce light loss as much as possible. Make sure that your cover glass, if you use one, is kept spotlessly clean. Except in the case of suspended metal halide lighting, keep the lights as close to the surface of the water as practical. This goes in the other direction too. Keep the water level in the tank as high as possible too. If there is free glass showing above the water line, light “spills” out through that area, whereas light that hits the walls below the water level is bounced back into the tank.