>>> does that mean adding CO2 would make a difference?
There is no 100% correct answer to that.
Generally it is the case that in lower light regimes with slower growing, (relatively), lower light demanding plants, CO2 injection does not give sufficient benefit to warrant the expense and/or hassle of it's use. There is normally enough CO2 coming into the tank from the fish and surface to supply the needs.
In a tank where there were no livestock to produce CO2 and no water circulation to move any CO2 dissolving at the surface, then it is quite possible, of course, that CO2 would be absent at the level of the plants. The thing is though, that is an unlikely scenario, and without circulation, how would your injected CO2 get to the right place?
In a very lightly stocked tank with minimal circulation, it is possible that CO2 could become a limiting nutrient in even a dim tank. In that case CO2 injection would help to a certain extent.
In an average stocked tank, (even a moderately understocked one), CO2 is rarely the limiting nutrient in lighting regimes of this type. The plants metabolism is just not fast enough to use the extra CO2 being introduced.
As an aside, although Vallis. species are generally regarded as lower light demanding, they do better with moderate light. The twisted cultivars, of which there are several, demanding more light then the straight leaved varieties.
