Lumens vs watts

> From: Whirlabout at aol_com
> Date: Wed, 8 Nov 1995 22:06:50 -0500
> This brings me to a question I've had - why are lighting intensities listed
> in watts per gallon rather than something involving lumens or candlepower?

It's difficult for the average aquarist to determine lumens since most
readily available bulbs aren't marked with lumens.  Even lumens would
not be adequate because there are so many variables.  To be really,
truly useful, you would need to say that "<some specific plant>
requires <some lux value> at the leaves with a spectral content of
<here's another problem>".  In essence, make sure you have the right
amount of the right light *at the plant*.  Since we don't know the
amount or type of light on a per plant basis and even more advanced
aquarists couldn't determine it if it was known, "watts per gallon" is
all we can recommend.