rules of thumb are idiotic. The wattage of light you need is completely dependent on how deep the water is, how spread out that wattage is over the surface and how much of that wattage is actually in the correct wavelength to be useful, how much of the wattage is actually hitting the surface, from what direction the light is coming from to reach the plants/photosynthetic organisms in the tank. All of these need to be close or the same as the setup the rule of thumb used to just blindly follow the rule of thumb. I highly doubt nano-reef tanks fall into that category.
To use a "rule of thumb" you have to know the conditions for which it originated, to get an idea of why it is what it is. They probably used a 50/55 gallon long tank since that is pretty much the standard. They probably used MH bulbs since that was also the standard. The tanks are also +12" deep and +12" wide and over 3' long. Tanks in that range need a wattage to reach the corners and depths with enough energy to still allow photosynthesis and good viewing light. With MH this is even more significant because MH are more point sources than T5 types are, meaning the light comes in at an angle for more of the surface area of the water than tube flourescent. Light at an angle of smooth water is reflected at a significant percentage. Light coming in at 90 degrees is only reflected at 3%. It's easy to see how in just changing the type of light from which the rule of thumb was made, makes it irrelevant. I'm not saying T5 is better than MH here, i'm just saying that the rule of thumb could only be made assuming light from one or the other, it can't work for both.
Next it's obvious to see that the water under the surface receives more wattage than the water at the bottom. With point sources of light, it's even more pronounced since the water directly under it would recieve much more wattage than the water radiating around it, even on the same depth. If your tank is restricted in any way to this area then the total wattage hitting your organisms is higher than the assumptions that the Rule of thumb makes.
The rule of thumb isn't that 6 watts per gallon is required for a healthy reef aquarium, it's that for the types of tank and lighting that the person used who got the rule of thumb going, that was the avg amount of lighting he needed to ensure that the reef would be healthy in all places of his tank. Much of the tank's volume received more light than what was needed, but none of the volume received less with his rule.
I'm not saying here that 6 watts per gallon isn't a good average. i'm saying that when you get to certain setups, especially less traditional ones, it's much more than what's needed, and possibly dangerously more for certain organisms. It's hard to think of 18 watts as being dangerously too much light when you hear people with 100+ watts on their 24g tanks. It all depends on what organisms they have as to how dangerous it is...but it very well likely is far more light than is required for them to be healthy.
Just look how bright the tiny tank is with even it's crappy stock lighting. It's not throwing colors like everyone else's because there is no blue. Get a 13, or if you must, 18 watt 50/50. It's guaranteed to get more light than what most reef's receive in the actual ocean. All the additional light is purely for your viewing benefit, not for the things in the aquarium.
In addition to that, a system 3 with liverock and substrate is significantly less than 3 gallons.
Sorry for the rant there, but rules of thumb really do cause more problems than they solve. And they're everywhere.
