Re: Lighting

The only truly important measurement of light is the amount of 
Photosynthetically Active Radiation (PAR) falling on your plants 
measured as 
quanta per area of plant per time interval. Neither Watts nor lumens 
measures this. Lux (Lumens per m2) comes closest but it doesnt 
discriminate PAR. This means that different lights will be more or less 
effective even though they have the same light output. Unfortunately, the 
average aquarist has no way to measure this and therefore most use the 
approximation of lumens if they can get it or Watts for the unwashed 
majority. Unless you can provide a cheap method of measuring uE/m2/sec 
then I suggest you not confuse the issue by denigrating the use of Watts 
or lumens.
Typically, submerged aquatic plants are considered shade adapted, which 
means that they have low compensation points and low Ps max values. A 
typical plant might have a compensation point of 10 to 15 uE/m2/sec while 
a plant such as Cabomba is as high as 80 uE/m2/sec. This level of 
lighting can be supplied by less than 1 Watt per gallon but plant growth 
would be slow. Ps max levels are in the 300 to 1000 uE/m2/sec range. If 
you pack the top of your tank with fluorescents then you will approach 
the lower end of this range. Intense metal halide bulbs can deliver 
light levels to 1000 uE/m2/sec and beyond, but as karen has mentioned at 
these levels you will likely need CO2 injection and a rigorous 
application of fertilizer.


PS. When I stopped working on aquatic plants in the laboratory after many 
years and set up a plant tank I had no idea how much lighting I needed 
because I was used to using a light meter and just adding light until my 
intensity was where I wanted it ... I wish that I had known the 2 to 4 
Watt rule as it would have saved me a lot of grief in setting up my home