The tank where I work has gravel and harder water. Java fern, water sprite, and several other plants that I can't remember the names of grow just fine without a bit of co2 or fertilizer. In this case, the plants were added well after the fish were there, already adding plenty of their own fertilizer, but if you are starting out, you could just use a fertilizer tab that is made for aquariums and just try whatever plants you like. Some may grow, some may not, but I don't think the gravel or lack of co2 should be an impediment. I also have a 10 gallon that has plants growing in gravel. The only thing I've had a problem with is the little tiny plants (sorry, again can't remember the name) that grow roots that are so tiny that they can't really grab onto anything and they float out whenever I bump them with the gravel vac. I'll try to post a pic so you can see what I mean.