With ordinary bulbs, whether frosted or plain, and a reflector of average efficiency, the following are the requirements of various tanks, assuming no help from even weak daylight:
Translated into more useful terms, the figures in the last column multiplied by 100 give the wattage to employ, assuming 10 hours per day of illumination. Thus, a 40-watt bulb turned on for 10 hours over a 5-gallon tank is satisfactory, giving 0.4 kilowatt-hour per day. Those familiar with the inverse square law may start to do a little figuring for themselves and wonder why a 45-gallon tank, with twice the depth of a 5-gallon tank, doesn’t need 4 times the wattage instead of a mere 2 1/2 times.
There is a twofold reason. First, the difference in plant depth is not doubled because the tank depth is doubled. Plants right down on the sand will be perhaps at about double the distance, but not the average plant leaf, which will often be able to reach just as near the bulb, whatever the tank depth. Second, the use of a reflector modifies the inverse square law and focuses the light into a beam or band projecting it down into the tank and not allowing it to spread out as much as the light from a naked bulb. It also so happens that, even without an overhead reflector, the internal reflection from the inner faces of the glass prevents the escape of much of the light and aids the illumination of the deeper parts of the tank.
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