Sure. But, even when I assume that the light source is floating on the
water surface, we are still *way* out: I get 3.7W per gallon for
a moderate to high light level tank.

It's probably not applicable for a tank the size of yours. But I think it's a
reasonable figure for "normal" sized tanks. Say, between 10 and 100 gallons.
Most people who want a "bright" tank aim for 3-4 wpg.

I have a 75 gallon tank. 48"x18"x18" (top of the gravel to bottom of light).
There's 250 watts of PCF light. Plugging in the numbers gives me a value of
0.18 for 11 hours...exactly what they recommend for a moderate to bright tank.
Which is what I would consider this tank to be. It's more than moderately lit,
but not really bright -- plants like Lilaeopsis grow slowly.

However, I'm not sure it's worth doing all the math; this formula is clearly
just a fancy way of getting the watts per gallon rule of thumb. There's a
linear relationship between volume and lighting (which a different Krib article
claims is breaks down for both very small and very large tanks).


Leigh

http://www.fortunecity.com/lavender/halloween/881/