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Watts and hogwash
The 2W/G rule of thumb is based on several assumptions -- all true. It is
still just a rough guide. The trick is to work on the things you can
control. The better you do them, the less W/G you may need for good growth.
It assumes adequacy in other nutrients and that they are "well balanced."
;-)
It assumes reasonable spectrum in the fluorescent (or MH) lamps, with lower
lumens often an indication of better spectrum (less high-visibility green
and more red and blue for better photosynthesis). [Some really bad lamps are
in the trade, but the big names like GE and Phillips have gotten very, very
good at getting high efficiency out of the phosphors for the entire life of
the tube. Typically about 90% of average when the tube no longer fires at
all.]
It assumes a fairly normal tank shape. [Extra long or tall hex tanks
probably need somewhat less W/G.]
I see several folks out there gagging on my last paragraph above. :-) It is
well-entrenched aquarium mythology that deeper tanks need more light to
"penetrate" to the bottom. That's pure hogwash, unless you have such dense
blackwater the back is hard to even see. Your plants would really look lousy
in any tank with *any* significant depth attenuation.
There is *no* effective attenuation of the photosynthetically active
spectrum in clear tanks that are less than a few *meters* deep. If you
really *do* need more light, it is because of the way your plants are
arranged, with big swords shadowing hairgrass, gloss. etc., or a *lot* of
algae on the glass absorbing/scattering the internal reflections. Arranging
plantings for low-light plants, like Anubias, to be in shade is something I
think I've heard Karen expound on. It's the cheapest increase in effective
light you can buy.
In free space, radiation falls off in intensity as the inverse square of the
distance from a limited source (point).
The tank is not free space, and your lamps -- even MH -- are not point
sources. Light that you manage to get coupled into the tank through the top
surface stays inside, like a light pipe, until it encounters some
absorbing/refecting surface.
If that surface is light-colored gravel, it may be mostly lost out into the
room. If it is plants, much green is lost, but much of the rest of the
spectrum is absorbed and used for photosynthesis. BTW, anything you can
actually *see* in the tank is from light not being used by the plants!
Essentially, what you need is enough light to properly illuminate the
planted acreage you have. Tall tanks need less light, per gallon, and low,
wide tanks need more, all other things being equal. 2W/G, efficiently
coupled, will raise most plants very well in standard-shaped tanks.
Surface loss at the water is minimal (typically about 10% under normal, low
high-reflective hoods) unless you let it overgrow with Riccia, duckweed,
etc. Likewise the losses of cover glasses are negligible (<10%) if kept even
moderately clean. Don't worry about it.
The experts using the lower light levels have all paid close attention to
the following.
Get the light as close to the surface as possible for other reasons.
Spillover is truly expensive. It rarely adds to the aesthetics, too, unless
you need the room light.
Skinny fluorescent tubes are more efficient at generating light than fat
ones. Going from 1.5" (T12) tubes to 1" (T8) is an improvement, with CF
being even better. Skinny tubes also permit design of more efficient
reflectors, so restrike losses are reduced, and angle of incidence of most
light is more vertical at the surface. [Grazing incidence light is somewhat
more reflected away from the surface or cover and wasted.]
Any dark colors inside the hood are serious sources of loss. Bright white
exterior paint is very good, but coated mylar and coated aluminum can be a
lot better with skinny tubes. [See the AH Supply web pages for more on this.
http://ahsupply.com/36,40,or.htm ] Plain aluminum is fair in blue and UV,
but absorbs up to 15% or more of the most useful photoactive spectrum in
green, red and near-IR. Nevertheless, I have a lot of commercial hoods lined
with household aluminum foil -- it beats that black or brown plastic hands
down!
Look to the design of the inside of your hoods or plantings to get the
required W/G down, and quit worrying about stuff that does not matter, like
depth and surface losses. <g>
Free advice is worth every cent, too. <VBG>
Wright
--
Wright Huntley, Fremont CA, USA, 510 494-8679 huntleyone at home dot com
To err is human. To blame someone else is politics.
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