Join Date: Mar 2004
Location: S.F. Bay Area, CA, USA
I use an All-Glass brand 12" 8W (T5) normal-output fluorescent strip light (designed specifically for a 2.5 gallon tank) and can grow just about anything with it. Anything more, and I get some nasty algae. Even with the 8W, I get nasty spot algae on my Anubias 'petit.' IMHO, people tend to go overboard with lighting nano tanks. It's one thing if it's just your preference to have an intensely-lit tank, but it's absolutely not necessary to grow a very wide variety of plants. Either way, my experience has been more light = more problems.
If you're going to use a 20W desk lamp, it'll be good to have it raised at least a few inches above the tank.
Regarding the 20W vs. 100W business... Basically, these "energy saver" bulbs are used to replace incandescent lights. A 100W incandescent is said to put out *maybe* 20W-worth of light, and the rest of the energy is heat. It's a major waste of money because your bulb is eating up 100W but you're only getting 20W of light. Compact fluorescent lights actually put out close to 20W of light and very little heat compared to incandescents. Now, when we talk of "watts per gallon," we're typically referring to normal-output fluorescent lighting. So these are mostly the T8's and T12's. It's said, however, that a compact fluorescent with good reflectors will provide up to 160% of it's equivalent normal-output fluorescent. So you could take the wattage of a power compact bulb - say 26W - and think of it as more like approximately 40W of light. Again, this would be with good reflectors. While this may seem very efficient, you have to take into account that the light *distribution* is not always the best, depending on the type of compact bulb(s) you're using.
Whew - that's probably more than you wanted to know. Sorry if I added to your confusion.