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**To**:**"'Aquatic Plants'" <aquatic-plants at actwin_com>****Subject**:**Re: Watts/lumens (again?)****From**:**Mark Fisher <Mark.Fisher at tpwd_state.tx.us>**- Date: Fri, 22 Jan 1999 14:16:22 -0600

>When we talk about two watts per gallon, are we talking standard florescents? Yes, we are talking about standard fluorescents AND standard sized tanks! For example, a "tall" tank would have less surface area per gallon than a "standard" tank, while a wide, short "breeder" tank would have more surface area per gallon. Each tank style would need different amounts of watts per gallon to achieve the same amount of light/surface area. I have a reference that may be helpful here: Moe, Martin. 1992. The Marine Aquarium Reference: Systems and Invertebrates. Green Turtle Publications, Plantation FL. He has an equation on page 159 that allows you to calculate how many watts of light you need for a given amount of lux you desire (remember, lux is lumens/square meter). This equation takes into account the efficacy (i.e., lumens/watt) of the bulbs you want to use, whether they are NO, VHO, or metal halide, and the surface area of your tank. The equation is: watts=[(lux)x(area or tank in square meters)] / [(bulb efficacy)x(utilization factor)] You supply the amount of lux you want, the area of your tank in square meters, the bulb efficacy in lumens/watt, and the utilization factor, which is the amount of light that actually strikes the water. Moe sets this to a constant, 0.5, although I suspect a good reflector can improve this. Anyone care to comment? For example, if you wish to have 10,000 lux over a 75 gallon aquarium (48 x 18 x 18 inch, or 0.56 square meters), and you intend to use 48" T-12 NO fluorescents at 79 lumens/watt, then watts = [(10,000)x(0.56)] / (79)x(0.5) = 141.7 watts. Of course, since 48" T-12 only come in increments of 40 watts, you would have to round up to 160 watts, or 4 tubes. This would put you a little over your target of 10,000 lux, but pretty close. It's a simple matter to rearrange this equation to give you lux for a given wattage and bulb efficacy. Note that this is the amount of light at the water's surface, not the bottom of the tank. Also note that this barely meets the 2 watts/gallon rule, but nevertheless is a fairly brightly illuminated tank. Why are all the best references targeted toward marine aquariums? Regards, Mark

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