"watts per gallon"
 

Most aquaria are 1x1x2 proportions. Put the gallonage up by a
factor of 8, the area to be lit goes up by a factor of 4 and
the distance to the bottom by a factor of two. Although the
light intensity falls off as the square of distance for a point
source, it falls off only as the distance for a line source
such as we use in aquaria. So, 8 times the gallonage does
match 8 times the light required.

For deep tanks (tall format), light does not fall off as much
as you would expect from the line-source rule compared to
1x1x2 format: the angle at which a lot of the light hits the
glass-air boundary on the sides at less than the critical angle,
so is reflected back into the tank.

Watts/gallon is a surprisingly consistent measure to use. My only
reason for enquiring about lamps used is to convert it to units
available to the 90%+ of the world that has access neither to
US gallons nor to US lamps.

"watts per gallon"
 

Simply use watts/liters.
..5 w/liter to 1w/liter.
You can have more or less and do fine also, but this is a good general
range and offers a fair amount of wiggle room to account for the large
variation.

Lighting is the biggest variable we have, the bulbs that are new in a
tank vs the bulbs that are 4 years old are certainly not the same. Many
things come into play.

I think overall, folks addressing the CO2/nutrients will get the most
out of their lighting, no matter what they have as long as they are in
the LCP range or higher.

I've found that larger tanks do quite well with less light also, I
think this is due to scattering and better angels of light strike on
the leaves, at a short shallow distance, the light only comes from one
very intense angle. These shallow tanks tend to have one buln, whereas
the large tanks have several, this gives a nicer spread of light.

I like HQI's but they do not produce the nice even spread of FL's
tubes.
So I use both and this gives me the best of both.

But that's very high light, so I turn on the HQI's for 3-4 hours and
then the PC's for 10 hours at low light.

Good write up John BTW on your site.

Regards,
Tom Barr

www.BarrReport.com