Thanks! I will try the sand thing as suggestedDon't worry, the same thing happened to me when I first bought HC. I think it just has an adjustment period to get used to your aquarium. Mine looked worse than yours before it got better. Then about a week after I planted it, it just looked GREEN. New roots were coming out and the yellow had faded. Make sure no other plants are shading it. I made that mistake and the shaded HC slowly lost leaves but it is slowly coming back.
I have mine under 42 watts of T5's. I dose flourish excel when I remember and Leaf zone weekly. I recently moved one light directly over the HC and its just going crazy.
Be patient and let it adjust to your tank. I would take this time to plant it how you want it to spread though. It will spread faster. Hope this helped.
After looking at your picture, your gravel substrate is not ideal for HC. It prefers smaller grained almost sand like substrate. Not to say your's won't work. It will just have a harder time rooting in your substrate. You could plant it how you want then spread some sand over the top of the plants. This will give the roots something to bite into when they first emerge.
What solution would you suggest?API Leaf Zone is worthless as a fertilizer. It's got a little K+ and Fe. It's disgusting that API even sells this product; an RO unit with old damaged filters would leave more nutrients in your water.
So, with that uglyness out of the way, I'd say your HC is turning yellow because of a lack of nutrients, specifically nitrogen. You're also letting it grow rather thick.
Yup, a slight reduction in light because 2wpg of CF over a tank that size is about where it takes good DIY CO2 and excel to keep the algae out. More light isn't so necessary as people think; good spread is.Philosophos, I think your over complicating things a bit. Why would you want to scale back lighting? Because of co2?