I have a tank of about the same size with 2 x 3ft flouros and no CO2 and I have 3 types of Echinodorus in there with runners and they like light. Leave the lights on longer--mine are on 17 hours/day. Fish give off CO2 and the plants can utilize it if you don't drive it off by aerating the hell out of it. Contrary to popular belief ambulia is not always easy to grow and it is not always easy to convert plants from emersed to submersed growth, and when you buy plants from the petshop they have usually been grown emersed (particularly the stem plants). I also have ambulia in this tank and it is growing but not flourishing. When you set up a community tank it is all a big compromise for fish and plant, what is good for one is not going to be ideal for something else. I tend to grow plants only where they like it rather than forever trying to change the environment. I only use added ferts (flourish excell) if I start to get algae problems, and once a year add JBL fert balls to heavy feeders.