Tropica had some research on their website that showed that CO2
addition can actually improve growth more in low-light situations than
in medium or high light.

This doesn't mean CO2 can make up for insufficient lighting. The
light needs to be at least enough for the plants to grow.
Chuck Gadd
http://www.csd.net/~cgadd/aqua

The research was already there awhile ago, several papers, one by
Jensen from 1989 in particular shows this clearly with the
assimilation irradience curves shown for various macropytes. Other
papers are also out there documenting this.
The Tropica site has it more distilled down for the hobbyist.
I take that another step further but if you go too simple, folks think
you are full of bull, too in depth and too much research mumbo, they
miss your point.
So a book is in order

Regards,
Tom Barr