This article is a short explanation of how much light is needed by plants, how to provide that level of light and what types of lighting to use. In summary, the hobbyist needs to provide 30–40 PPFD of light at substrate depth to maintain a planted aquarium. For a 0.9 x 0.32 x 0.38 m aquarium with fluorescent lighting this works out to be about 39 W (135 W/m2) of lighting. For LED lights, two 5050 SMD tank-length strips would provide adequate light. For LED chips and arrays 2.6 times less wattage is required than for a fluorescent lamp (15 W over a 0.9 m tank). Any lamp would suffice but some are better than others. You can provide too much light. For maintaining a planted aquarium the levels of CO2 are more critical than the amount of light once the minimum light demands of the plants are met. How much light do I need to grow plants? Different plants require a different minimum amount of light to grow. Consider the following table: The light is measured as PPFD (photosynthetic photon flux density) in the units of mmoles/m2/s and is the measure of how many photons of light, that can drive photosynthesis, are striking a given area every second. This value of PPFD refers to wavelengths of light that plants are photosynthetically sensitive to. Below is the action spectrum of Elodea densa (Figure 1). An action spectrum is a measure of photosynthesis at different wavelengths of light . As you can see, Elodea photosynthesizes best in the blue and red spectra. Conversely, our eyes are more sensitive to the green and yellow wavelengths than the red and blue which is why measures of Lumen and Lux (lumens per square meter) are not used as these measures are based on the quantities of yellow and green light. The wavelengths over which photosynthesis take place is often referred to as PAR (photosynthetically active radiation); and PAR and PPFD are often used interchangeably by people who don’t know better. PAR is an absolute value while PPFD is relative. You can have 10 PAR, that is 10 mmoles of photosynthetically active photons, emitted from a lamp and if you concentrate these 10 PAR over an area of 0.5 by 0.5 meters you would have 40 PPFD. Some plants need a lot of light while other don’t need as much to grow. Low light plants need only 11 PPFD while highlight plants may need as much as 60 PPFD. Most plants need about 20–30 PPFD. It is generally accepted that 30 to 40 PPFD is enough light to grow any plant . The problem however, is that this 40 PPFD must be at the lowest level of the tank, at substrate depth, if you plan to grow carpeting plants such as Glossostigma. The intensity of the light changes with water depth. This is due to two properties of light. The first is the Inverse Square Law which means that light intensity decreases exponentially by the square of the distance from the light source. A point source of light, such a light bulb, will lose light intensity exponentially as the distance from the light source increases (to be precise, by the square of the distance from the source). The other property of light is that it is absorbed by the water, and chemicals in the water. This loss of light intensity is called attenuation. Both of these are occurring in your fishtank but the major factor is attenuation. As a consequence of internal reflection of the light, as it moves through the water and reflects off the glass, a lot of the light that enters the aquarium is trapped in it. When one examines how light intensity diminishes with water depth (Table 2) it is easy to see that the inverse square law (which would have an attenuation coefficient of 2) isn’t the major contributor to light loss. In the addendum add: List of websites from which PPFD data was obtained to calculate the attenuation coefficients. http://blog.aquanerd.com/2010/02/ecoxotic-panorama-module-par-numbers.html http://www.plantedtank.net/forums/archive/index.php/t-379417.html http://blog.aquanerd.com/2011/08/current-usa-truelumen-pro-par-numbers.html http://www.plantedtank.net/forums/showthread.php?t=415522 http://www.advancedaquarist.com/2013/3/review http://www.barrreport.com/showthrea...who-knew/page8?highlight=lighting aqua forest http://www.plantedtank.net/forums/showthread.php?t=415522 http://www.plantedtank.net/forums/showthread.php?t=415522 http://www.plantedtank.net/forums/showthread.php?t=415522 Average of planted aquariums The first three values are for tanks with pure, clean water. The last two are from tanks with water that has been tinted brown by dissolved organic acids such as tannins and phenolic acids etc… These compounds absorb blue and red light. Water by itself absorbs red and far red light more effectively than other wavelengths. What this data tells us is that clean water absorbs less valuable light. The cleaner you keep your aquarium’s water the less light you need to beam into your aquarium to achieve 40 PPFD at substrate level. How much light do I need over my aquarium? The old of idea of Watts per gallon does not hold up to scrutiny and I will make no more mention of it. I prefer to think in terms Watts per square meter as a concept for working out how much light to put over an aquarium. The reason for this is because we measure light as a unit of photons per square meter. The wattage of the lamp is strongly correlated with its light output . There are two critical issues to consider. The first is the wattage of the light source and the second is whether or not you are employing a reflector. A reflector is unnecessary with LEDs but if you are not using one with T8 or T5 tubes (and those tubes are 10 cm or more above the water surface) then only 32% of the light is actually directed down onto the water (Figure 2). And with such a wide arc of 114° only 83% of the light is actually entering the water. So, for every 10 W of light over the aquarium, if you aren’t using a decent reflector, only 2W of light is actually entering the water column. If you decide to fashion a simple reflector out of cardboard and aluminium foil then you can expect about 70% reflector efficiency (this assumes dimensions of 5–7 cm wide and the lamps being 2.5 cm deep into the reflector ). For the following explanation I will assume such a cheap, do-it-yourself reflector. Expensive, professional reflectors are not very much more effective. The next matter is the wattage. The average tube light produces about 0.9 PAR/W (0.87 for T8s and 0.93 for T5s) . Assuming a cheap DIY reflector and an arc of light that is not more than 114°, that would mean a maximum incident angle of 57°, approximately 53% (10 W x 0.9 x 32% x 83%) of your light energy is entering the aquarium. The light entering the aquarium will dim by 5 to 11% per 5 cm depending on the attenuation coefficient. (The 5% is based on the 0.95 coefficient of an actual aquarium. See Table 2.) This means, to attain 40 PPFD at a depth of 35 cm, with an attenuation coefficient 1.624, you would need 70 PPFD just below the water level. This would be 120 PPFD at the water’s surface. This equates to 135 W/m2 of fluorescent tube lighting. For a tank of 0.9 x 0.32 x 0.38 m, this is 39 W of light. To estimate how many Watts per square meter are needed over the aquarium, see Figure 3 below (to be safe it would be better to estimate one’s light wattage based on the average attenuation). How to use Figure 3 is easy. Let’s assume you have tank that is 1.2 m long, 0.45 m wide and 0.45 m deep. You have a reflector of 70% efficiency with a beam arc of about 60° (83% transmission) and your lights deliver 0.9 PAR/W. You would be planting plants at a depth of 0.4 m. At this depth, and if you had an average attenuation coefficient of 1.624, that means you would need about 150 W/m2 over the aquarium. Your tank’s surface area would be 1.2 x 0.45 = 0.54 m2. So, to calculate the total watts of light you would need: 150 W/m2 x 0.54 m2 = 81 W. Indeed, this is how much wattage I have had over a tank 5 cm deeper than the one just assumed and in that tank I have grown all manner of plants from Vallisnaria to Potamogeton schweinfurthii (see Figure 7 at the end of the article). What about LED lights? LED lights behave differently to those of tubes as they are, essentially, point sources. LEDs come with built in lenses that create a beam arc of 60°. At this beam arc their light follows the inverse square law perfectly until they enter the water column. At an angle of 60° 82% of the light will enter the water but no light is lost due to poor reflector efficiency. As such 2.3 times more light will actually enter the water column compared to a tube lamp without reflector. For tubes with a reflector, LEDs are 1.4x more efficient at getting light into an aquarium. For LEDs you could take the W/m2 estimate from Figure 3 and divide by 1.4. So, for the same 0.9 x 0.32 x 0.38 cm tank, you would only need 28 W. There is still some uncertainty with respect to this estimate. While behaving as point sources of light, most LED lights come as arrays or a sequence of LEDs on a strip. Because PPFD (and Lux) are relative measures these strips can amplify the PPFD, as illustrated in Figure 4, and yield much higher PPFD that what can be predicted by simply dividing the PAR/W output by the area the light will cover. There will be greater amplification of the light the longer the LED array and the greater the distance from the water. However, with greater distance the more light intensity is lost to widening surface area. For an 5050 SMD LED strip with LEDs every 16 mm, there is another level of overlap per cm from the water’s surface. (The beam arc of these LEDs, at a distance of 10 cm from the water’s surface each LED will light a diameter 34 cm.) Based on measurements  of a single Elite White  strip running at 9.6 W per meter (0.8 m of LED was used) 28 PPFD was produced at 10 cm from the light source. To achieve 30–40 PPFD at a depth of 35 cm a surface PPFD of approximately 56 PPFD is needed. Two such strips would be needed to light the aquarium to a depth of 35 cm. This is 53 W/m2 of LED light from 15 W of LED light (0.8 m of 9.6 W/m times 2). LED strips are 2.6 times more efficient at getting light into the aquarium than T8 and T5 tubes. Proper LED arrays, running at lower currents produce 1.5 times as much PAR/W as LED strips and are consequently more efficient and could, therefore, be 3.9 times better than tubes are at lighting aquariums. The avid Do-it-yourselfer preoccupied with efficiency would be better served buying 1 W LED chips and building their own array rather than buying LED strips. These LEDS are far more efficient with respect to energy consumption as well as PAR/W ratio. It would be safe to estimate the W/m2 from Figure 3 and divide that estimate by 3.9. (So 39 W would become 10 W.) Can your provide too much light? Yes! It is possible to provide too much light. Many experimental measurements have now been performed and plants can become saturated with light. From approximately 34 PPFD the average plant will grow less responsive to light . From approximately 238 PPFD the plants are only half as responsive to light as compared to lower PPFDs. What this means to the aquascaper is that increasing the light will not necessarily result in better growth. What is more is that you are actually wasting energy by increasing the light much beyond 34 PPFD. If the aim is 30 PPFD at a depth of 35 cm, then in the top 10 cm of water the PPFD would be 46 PPFD and the plants would already be only 90% as responsive to the increased light as compared for the lower energy input. The analogy of the light being the accelerator to growth and fertilizer the fuel, what this means is that you get more efficient use of the fuel at lower PPFDs than at higher PPFDs. As you increase the light above 34 PPFD more of that light is wasted as the plants become less responsive to the light. At 238 PPFD the plants are only 50% responsive to the light and you any increase in light input is 50% wasted energy. Increasing the light will increase the stress on the plant as the photons, instead of producing energy for photosynthesis, will instead cause the generation of free radicals that will damage the plant tissue. What is worse is that algae benefit from higher light levels than plants. As the plants begin to suffer under higher light the algae will do better. In an environment full of fertilizer, such as what a planted aquarium is, the algae can quickly get out of hand. Generally, PPFDs of up to 200 in the top 10 cm are well tolerated by planted aquariums but then care must be taken to ensure high and constant levels of CO2. The more light is beamed into the aquarium, the more sensitive the aquarium becomes to imbalances in CO2, nutrient demand and light; and this makes algal outbreaks more likely. I have seen beautiful tanks running at more than 400 PPFD but these tanks, if you miss a water change or make an error in fertilization, quickly become algal soups. The application of more light than the plant can use is the key to getting red plants. The red of plants is due to anthocyanins. These plant chemicals absorb UV, blue and green light and fluoresce the light back as red.  The production of these chemicals is governed by the activation of Cryptochrome plant pigments (that are similar to Chlorophyll but have beta-carotene at their reactive centers instead of iron) that sense UV, blue and green light, and initiate the production of anthrocyanins. An alternative route to red plants is to reduce the nitrate levels. This will also stress the plant as it won’t be able to make new protein and the molecules needed to harvest and transport the energy absorbed from the photons. The plant will, consequently, grow slow and need to absorb less light. The plant responds by reducing the amount of blue and UV light that can reach the plant’s tissues (and damage the proteins, membranes etc..) and in so doing reduce its energy input to match its metabolic needs. In short: Anything less than 20 PPFD at substrate depth is a low light aquarium and anything above 40 PPFD is a high light aquarium. 30 to 40 PPFD is a “just-right-light” aquarium where the plants will grow well if they have adequate CO2 and nutrients. For low to “just-right” light aquariums no additional CO2 supplementation would be needed if the KH is retained at 4 or above . What type of light to use? Much advice is dispensed concerning the Kelvin rating of the tube, or its color or spectrum. Much of this nonsense. The Kelvin rating is largely irrelevant. What matters more is that the spectrum of the emitted light match as closely as possible that of the action spectrum shown in Figure 1. This action spectrum differs very little from plant to plant. You can grow planted aquariums with Cool White and Warm White fluorescents as they emit large amounts of red and blue light needed for photosynthesis. Daylight compact fluorescents are very popular as well. That being said, some lamps may be better than others. Popular lamps are the Sylvania Gro-lux and Aquastar; the Arcadia Plant Pro and Giesemann Powerchrome Aquaflora. These lamps tend to be more expensive than the standard Daylight and Skywhite fluorescents that can be bought at most hardware stores. In addition to the chlorophylls there are two other plant pigments: cryptochrome and phytochrome. The former is stimulated maximally at 430 nm and the latter at 680 nm. The stimulation of the former results in the production of new chloroplasts (along with other important plant functions) and the latter the elongation of stems and production of new leaves. If the light source is deficient in violet and far red wavelengths then the plants will not grow optimally. The more expensive plant tubes are designed to stimulate these plant pigments and in so doing stimulate plant growth. It is advisable that the excited new aquascaper invest in a good CO2 supply and substrate before investing in expensive lights. The plants will grow perfectly fine under standard fluorescents (or white LEDs) but will suffer from a poor substrate and a shortage of CO2. CO2 is far more critical than lighting. In fact, the tendency to invest heavily in light over CO2 almost always results in a tank overgrown with algae and a disenchanted aquascaper leaving the hobby. References 1. The Complete Book of Aquarium Plants, Robert Allgayer and Jacques Teton (1987, ISBN 0-7063-6614-X) 2. Data extracted from Figure 8.8 of Life, Science of Biology by William K. Purves, David Sadava, Gordon H. Orians, H. Craig Heller (6th Edition) Sinaur. If anyone has the original reference (and the original data) please let me know! 3. Tom Barr, http://www.barrreport.com/showthread.php/4902-ADA-lighting-at-Aqua-Forest-and-nice-low-PAr-values-who-knew?highlight=lighting aqua forest 4. Cor “Greystoke” DeWit, http://www.apsa.co.za/xenforo/threads/light-spectrum-data-base.4454/ 5. Cor “Greystoke” DeWit, http://www.apsa.co.za/xenforo/threads/the-efficiency-of-a-light-canopy.4379/page-2#post-41654 6. http://www.apsa.co.za/xenforo/threads/wolfs-led-diy.11072/ 7. http://www.led-lights.co.za/lumi-shop/waterproof-flexible-strip-light/ 8. Binzer, Thomas, Kaj Sand-Jensen, and Anne-Lise Middelboe. Community photosynthesis of aquatic macrophytes. Limnology and Oceanography 51.6 (2006): 2722-2733. 9. Merzlyak, M. N., Chivkunova, O. B., Solovchenko, A. E., & Naqvi, K. R. Light absorption by anthocyanins in juvenile, stressed, and senescing leaves. Journal of experimental botany 59.14 (2008): 3903-3911. 10. Dirk “Prof” Bellstedt, http://www.apsa.co.za/xenforo/threa...-with-low-carbonate-hardness.2152/#post-19497 Addendum: A graph to quickly estimate how much light is left at a certain depth. Note that a tank with water rich in organic chemicals (that tint the water brown or yellowish) will have lost 50% of the light by a depth of 25 cm while a tank with clear water will still have 50% of its light at 70 cm. The average tank would retain 50% of its light up to 40 cm of water. Keep your water clean. Figure 7: This is an old aquarium of mine with nothing but two 40 W cool white fluorecents over it. There is no CO2 or fertilization. While not visible, there is a lawn of Echinodorus tenellus as well quadricostatus growing there at a depth of 40 cm.