Definitely a COOL experiment. 8) Take a few small clippings from your main tank and throw them in there too. Have you tested the water column for nitrates and phosphates? Even though its filled with tap water, the soil nutrients ought to leach into the water column somewhat.
The 15 watt bulb should be enough light if the tanks not overly deep. 3WPG isn't too shabby.
The only thing that may cloud your experiment is the lack of CO2, especially when comparing your exhisting clippings to your main tank.
I'll be very interested in your findings.