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Lighting: the WPG rule

> From: Tim 
>Subject: Lighting "rules of thumb" 

> 2 to 3 watts / gallon is the often repeated rule of thumb for
> lighting for planted freshwater aquariums. I struggle with that a
> bit given that going from 2 to 3 represents a full 50% difference. A
> 50% variance in most other aquaculture metrics would surely mean the
> demise of the organisms. 

I disagree.  50% more food or fertilizer or CO2 will not kill plants in the
"acceptable" range.  They just grow more slowly or have less algae blooms.
Some plants are more tolerant of range than others.  Java Fern, for instance,
grows in the lowest light (0.5 wpg) all the way to very high light (4+ wpg).
Others are more sensitive.

> Also, it seems that tank surface area and
> possibly depth would be much more relevant than tank volume. 

True, as well as how tall the plants grow, how far from the water the
lights are kept, how reflective the hood is, and how well it focuses
the light into the tank, how much suspended algae and other particles
are in the water column absorbing light as it passes through the
column, and how much algae and crud you have growing on the tank sides.

Unfortunately, it is not easy to easily categorize each of these
factors, so a good starting point is "1-4 watts per gallon of
fluorescent light".  Once you've tried a particular setup, you can add
or remove lights to taste.  But the "rule" is surprisingly good for
most tanks.  It's very good at pointing out that the strip lights
provided in most tanks are inadequate.

> And further, is wattage necessarily the appropriate measure? 

Again, same deal.  The best measure is PAR, the Photosynthetic Active
Response (did I say that right Steve?), which is light power weighted by
the photosynthetic "spectrum".  Unfortunately, most lights have their
power measured in Lumens, which is weighted towards the human vision spectrum.
So again, it's almost better to go with watts for fluorescent tubes.

> For
> example, aren't compact flourescents supposed to provide so much
> more light for a given wattage than standard flourescents? Am I
> misunderstanding something on that one? 

Possibly.  Depends on the type of compact fluorescent.  The 9 and 13
watt CF's are no different from normal fluorescent tubes.  They even
use the same magnetic "tar" ballasts.  The newer "high efficiency"
CF's (27, 36, 40 watt, etc) are 50% more efficient if run with an
electronic ballast, kind of similar to high efficiency T8's.

Now, in comparing non-fluorescent lighting, there's a DEFINITE correction

> Of course, I understand that
> a "rule of thumb" is meant to be a rough basis and that the specific
> parameters of each situation need to be considered, but has anyone
> given thought to developing a more concise guideline for estimating
> the appropriate lighting for a given scenario?

People have tried.  This thread has been debated (see the Krib for instance,
under http://www.cco.caltech.edu/~aquaria/Krib/Plants/Tech/ for some
examples).  In the current FAQ on plant lighting, there's a
complicated calculation that adds into account the distance from the
light to the tank and the number of hours in the day, but it really
ends up saying 1-4 wpg.  In doing research for the new revision of
this FAQ, I am collecting data points to determine just how wide the
dispersal is when using the wpg rule... it's not too bad, though I might
make an upward adjustment for smaller tanks.

   - Erik

Erik Olson				
eriko at wrq.com