How was the HC purchased? Emersed or Submersed? It could be going through an acclimation period if purchased emersed.
Have you tested for nitrates? It could be a nitrogen deficiency. In one of my newly cycled tanks, I had almost no stocking in that tank.. therefore, my nitrates were 5ppm. I was also growing the HC with Flourish Comp and I was advised to use a better and more comprehensive dosing regiment. So I started using the GLA Dry Ferts Pack and I starting mixing the ferts myself using the PPS-Pro dosing method. This mix has Potassium Nitrate (KNO3). I'm not quite sure if that was the culprit from getting my yellowing HC to a nice green again, but after I started PPS-Pro it really started to take off.
And lastly... yes, it could very well be CO2! HC loves intense lighting and co2. I'm just not sure if Excel can be a true substitute on that one.