How large is your tank? I see a couple of issues with your set up. If all the light you have is a l7W bulb, you're lacking light. Anubias are about the only thing I would recommend trying with that kind of lighting. Don't fertilize too much, because you don't have the lighting to drive the plants to grow.
Yes, running an air stone will drive off CO2, and you do not want that to happen, especially if you're not providing any to the plants. Air powered sponge filters are not the best for planted tanks. Remember that Vals and Excel (liquid C source), do not mix well.
I have no experience with Marimo balls.
You might want to read this
over, it's very informative.