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I don't subscribe to the notion that light requirements can be broken down simply into ``watts-per-gallon'' or similar notions, because of the complexity of the problem. Some of the factors influencing the amount of light you need include, but are not limited to:
The first, at least, can be summarized easily: fluorescent and metal halide bulbs have similar efficiencies (light energy output per watt of operating power), so their ``wattages'' may be compared as roughly equivalent. Incandescent light, however, is about 1/4 as efficient as flourescent, so to convert requirements, think of 100 watts incandescent as equivalent to 25 watts fluorescent.
So now that we've established that it's impossible to give a perfect formula for your exact lighting requirements, let's try to do it anyway, just for fun. :) In the graph below, we see the results of many planted tanks' lighting, plotted in watts vs. liters (gallons shown on the alternate axis). The green squares are data from Amano's Nature Aquarium World series, and the red triangles originate from a survey of folks on the Internet. The few black circles represent typical single-strip lighting fixtures sold with the appropriate tanks. All data points use either fluorescent or metal halide.
First, it's worth noting there is a repeatable trend, around which the wattage varies typically by a factor of 2 in either direction. However, attempting to fit a ``watt per gallon'' (shown by the yellow dotted lines) does not really work well, especially for the Amano tanks. For the smallest sizes, 8 watts per gallon is too little! For the larger tanks above 100 gallons, 2 watts per gallon is too much.
Perhaps a better ``formula'', shown by the dotted blue lines, is obtained by calculating watts per ``estimated surface area'' (calculated by taking volume to the 2/3 power). This might seem a strange calculation at first, but when you stop to think of it, it does make sense, as the light falls on an area, not a volume. Note that the actual surface area depends on tank shape and all the factors I mentioned in the first paragraph, so that's why I'm calling it an ``estimate''. And finally, remember that these results are not saying it's impossible to grow plants at lower light levels; in fact you'll even notice a few data points in our survey that lie below what I've called the ``low'' cutoff.
As this is not a calculation most people want to make, here is a table summarizing popular values:
Amano, Nature Aquarium World Volumes 1-3, TFH Publications. (raw data: volume 1, volume 2, and volume 3).
Original data taken from archived posts on The Krib, personal acquaintances, and a survey posted on Usenet in 1998 (raw data).
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