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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 type of bulb (fluorescent, metal halide, incandescent)
- The brand of bulb (Vita-lite vs. cool white)
- Reflector in hood, distance from hood to the tank
- Shape of tank, reflectivity of walls
- Types of plants, degree of shade from tall cover
- Water chemistry and clarity (peat turns the water yellow and reduces light transmission)

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:

watts/m^{2}
| 5 gallon 20L | 10 gallon 40L | 20 gallon 80L | 50 gallon 200L | 100 gallon 400L | |
---|---|---|---|---|---|---|

low | 200 w/m^{2}
| 15 | 24 | 38 | 69 | 110 |

medium | 400 w/m^{2}
| 30 | 47 | 75 | 137 | 220 |

high | 800 w/m^{2}
| 60 | 94 | 149 | 274 | 440 |

References:

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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