Yes on the last two questions; carbon adsorbs [I didn't mis-spell that, it is adsorbs not absorbs
) some nutrients. As well, live plants do the job the carbon is doing with other substances better, so leave it to the plants.
On the light issue, adding another fixture, or getting a fixture with dual tubes, is the only option to add more light. But before doing that, consider different plants. I use minimal light and simply avoid plants that can't manage. Brazilian Pennywort is a stem plant that will do well with less light than most stem plants, I have it. Most of the sword plants also manage fine; the red leaf species are less likely, they need more light being red, but the green leaf species are OK.