Incadescent bulbs are not the best lights for growing plants. They generate too much heat and are not powerful enough to properly light a tank. If you could find a Compact Fluorescent bulb to use instead, that wouldn't be perfect, but it would be better. I know it's a 5g tank, but could you give me the dimensions? I could help you out better that way.
llj
EDIT: To answer your initial question. WPG doesn't work well for plants smaller than 15-20g. Smaller tanks tend to require more light to grow plants. For example, I can grow plants great at 1.4WPG in my 20g, but to grow the same plants in my 8g, I need 3WPG, in my 2.5g, I need 5.2WPG. All three of these tanks can be considered low-moderate lighting. There are a few calculators out there, but gaging lighting for a nano is still a challenge. I can guesstimate, and personally, if it were me and I wanted the same results that I'm getting now, I'd aim for 3-4WPG, since a 5g is somewhere between 2.5 and 8g. And this would only be to grow the plants that I'm currently growing under similar conditions (For me, that's an inert substrate, no CO2, high fishload, no ferts), but for others this is completely different. Knowing more about your setup and your goals for it would help out. Check out some of the journals here (mine are in my sig) and decide what you want to do from there. Aponogetons are pretty tough and you should be able to get it to sprout with the lighting you have now, but they will need an upgrade to continue to thrive.
llj
