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2 Watts/Gallon -- It's just a rule of thumb.
Someone noted recently that he actually measured an AH PC light as twice as
effective as another one, but that he had a hard time believing it.
AH Supply only claims about a 160% factor over conventional lighting
<http://www.ahsupply.com/36,40,or.htm> , but the reflector design, the
reflectivity of the surface, and the size of the lamp vs the area of the
reflector all have some effect. Spectral distribution of the light may be a
serious factor, too.
I have always liked the 2W/G of NO T12 fluorescents in a normal aluminum
(85%) or white (90%+) reflector as a good rule of thumb. A 40W (5000K) AH
"Bright Kit" gave me well over twice as much as I was expecting from the
Wattage alone. [My guess is that the tank would have cried for CO2 at about
0.75W/G! I had 2.7W/G!]
After the shock of the rapid algae growth and nutrient "instant imbalance,"
I would suggest anyone just getting going stick to about 1W/G of
AH-Supply-style lighting. Most plants will grow well, and the system will be
a bit more forgiving, IMHO.
A good, bright (MIRO-style) reflector with T8 lamps might go OK at about
1.5W/G, while T12 lamps can still be used at 2W/G. [This assumes that bulb
crowding is causing considerable restrike, which is a major source of loss.
See the AH site referenced above.]
Rules of thumb are great, but I think a lot of folks here may have had
trouble getting going by trying to do much at first -- particularly with the
amount of light.
Excessive lighting alone has a tendency to make algae an uncontrollable
plague, and algae-coated plants simply don't do well, IME. Start a bit more
cautiously and grow your techniques more gradually. "Bright Kits" are
compact, so you can usually add more, later, when ready to go to higher
Just my $0.02.
Wright Huntley -- 650 843-1240 -- 866 Clara Dr. Palo Alto CA 94303
Nobody every recommended a dictatorship aiming at ends other than
those he himself approved. He who advocates dictatorship always
advocates the unrestricted rule of his own will. Ludwig von Mises