Plants use iron to produce chlorophyll, so a lack of iron results in a lack of chlorophyll, and thus yellowing of the younger leaves. So basically it will make you plant greener and healthy. Flourish excel is a CO2 substitute. Plants use CO2 for photosynthesis, low CO2 will slow down your plant growth. I did not want to invest in a CO2 system so I started using Excel since it is a liquid CO2 substitute. I later started using Metricide14. It is the same base component (glutaraldehyde) as excel but has to be diluted to be safe for you aquarium. I would not start using that until you have researched and learned all you need to know about it.
Hmm, the iron might explain why my plants are still growing but turning yellow. Since you know more about this than I do, could the light be connected to plants growing but turning a yellow/reddish brown?